Dee Dee seems to have a thing for rap music and hip hop culture. In one episode she wants Dexter to see a dance she learned on Soul Train, but what made me realize it was in the Series Fauxnale "Last But Not Beast" she sings a line from Queen Latifah's song U.N.I.T.Y.
Mandark could apparently read Dexter's mind in his first appearance, but never displayed this ability again. However if you pay attention, all he actually did was tell Dexter that he could read his thoughts before Dexter asked. So the simplest explanation was that he was just bluffing and made a lucky guess as to what Dexter was thinking.
Or, maybe he was smart enough to calculate what he was most likely thinking.
This is actually confirmed if one pays close attention. When Mandark asks Dexter to set him up on a date with Dee Dee, Dexter agrees only after he realizes that Dee Dee will likely wreck Mandark's lab. We, the viewers, see this represented as several smaller Dexters speaking to the main one, but it's clear that it was supposed to represent Dexter's thoughts. If Mandark was truly able to read Dexter's mind, he wouldve known Dexters ulterior motive in agreeing to his request. The fact that Mandark acts completely surprised when Dee Dee does (unwittingly) wreck his lab shows that he had no idea it was going to happen.
From the same episode, Mandark appears to surpass Dexter in every way, though his dorky, easily duped self quickly appears by the end of the episode. From that point on, Mandark is either an equal or inferior to Dexter. Remember however, this was not Mandark's true first meeting with Dexter, this was revealed to be earlier where Dexter brushed him off and laughed at his first name (this is likely a retcon, but Dexter not even remembering some inferior intellectualis COMPLETELY in-character for him). From that point on, Mandark swore to ruin Dexter where it hurt; his science. This explains why Mandark, even at that early point, took perverse pleasure shutting down Dexter's lab, and why his mystique collapsed so quickly. While he was able to put up an impressive facade for a short while to spite Dexter, ultimately his insecurities betrayed him.
Mandark having a disdain towards his Hippie Parents may not be just because they neglect his love of science. Given his nickname and the look of his lab, Mandark can strike one as something of a Goth. An obscure but occasionally seen stereotype is that goths aren't too fond of hippies (what with the bright colors and lack of edginess). Therefore, part of this conflict could derive from difference between Mandark's interests and aesthetic and that of his parents.
When watching Dexter's Laboratory, if you think about it too much, you'll realize what a cruel, destructive Jerkass Dee Dee really is. When she's not completely destroying her brother's life's work, she at times openly belittles, insults, humiliates, and abuses him for shits and giggles.
It also implies that she's crazy. In one episode ( the episode with Dexter having to read to her) we see what goes on in her head. Not to mention suffering from eye problems (she once mistook a box for Dexter).
Look at how advanced Mandark's lab was compared to Dexter's and see how fast Dee Dee trashed it, and you wonder how much Dexter could have accomplished if he wasn't forced to rebuild every... let's be generous, week or so.
The opposite also counts: Many times, Dexter can come off as cruel to Dee Dee without even trying to understand his sister and just considering her an idiot because she doesn't share his passion for science. It seems likely, to some viewers, that they will drift apart when they grow up.
In the musical episode, one of the things Dexter makes is a helmet which Dee Dee destroys WHILE HE'S WEARING IT, by THROWING IT (and her brother who at this point is apparently still in diapers by the way) on the floor with enough force to shatter it. what would their parents think if they found out about THAT little "accident"?
There's that "Dollhouse Drama" episode where Dexter shrinks himself down to doll size, but the procedure makes his imagination run wild, so that he believes everything Dee Dee says in her game of dolls is true (i.e. that her Darbie doll is his wife). We keep switching between his perspective (wherein the dolls live, breathe and speak to him) and reality. Finally (from the real-world perspective) he tears the action figure who's supposed to be his Evil Twin limb from limb, and Dee Dee announces, "Dexter has broken his brother into pieces!" What's Dexter seeing??
He is seeing things as he imagines them, not as they would really happen. He wouldn't know what it looks like to rip someone apart, so he would see his Twin's limbs coming off painlessly.
Much of what Dexter does in his laboratory would be disturbing if it weren't so silly and cartoony. For example, in one episode he saws open his sister's head without anesthetizing her in any way, yanks out and casually discards her brain, and replaces it with one of several human brains he has in his lab (which raises more unfortunate questions).
There's been a few episodes where Dexter acts in a much more standard Mad Scientist way before the episodes hijinx starts, laughing to himself about how "the fools" don't suspect anything, Show Them All, etc. There's a chance that For Science! isn't his only motivation...
At the beginning of the race in "Mock 5", we see there were a bunch of others racers besides Dexter and Mandark. At the end Dexter comes in first and is immediately followed by the wave of lava rolling down the hill. So what happened to the other racers?
"Trapped with a Vengeance" has the villain Yohnny the Janitor terrorizing Dexter for making him work overtime and causing him grief for leaving all kinds of messes every day before leaving school. But... while he is sympathetic, Jerkass Has a Point here. Yohnny has to clear away all kinds of dangerous chemicals that Dexter is leaving in a mess! Not only does he have no training to properly dispose of or knowledge of these chemicals, Dexter doesn't label anything so one mistake could cause an explosion or release a poisoned gas. I know that Dexter is meant to be a Boy Genius but if any member of the scientific community saw that they'd heavily condemn Dexter. If a teacher saw that, Dexter would be banned from the school labs!
In "Dexter vs Santa's Claws" throughout the episode he believes that Santa Claus is his dad in disguise. So basically this means that Dexter's willing to murder his own dad just to prove his theory that Santa is his dad in disguise!!!
From the movie Ego Trip, as much as Dexter travels to the future, we never see Dee Dee. Not even once. It's rather easy to come up with reasons for this (such as her dying and this being partially why Dexter becomes a paranoid, scared man who never stands up to Mandark).
Otherwise, consider Mandark's feelings for her, what he became in the future and this can get really dark really fast.
While one could argue that Dee Dee's exclusion from the future events might have something to do with the fact she hopped in the time machine at the beginning, its also likely Dee Dee became as broken as Dexter did after Mandark became the Executive of the Corporation. While Mandark wouldnt do anything to harm Dee Dee (shes the object of his obsession, after all), it wouldn't change the fact that hes still the sadistic, slave-driving boss that regularly torments her brother via public torture. For all their jerkass moments to each other, Dexter and Dee Dee do care for each other in some way, so what's to say that Dee Dee wasn't emotionally broken by watching her brother getting tortured, and couldn't do anything about it out of fear for Dexter's safety?
It's even worse than you think; unlike Dexter, Dee-Dee wasn't shielded from the effects of the corrupted Protocore! She was most likely lobotomized like the rest of humanity, turning her into a barely-sentient moron. Sure, the show always did play her lack of common sense for laughs, but not to that level! And, building on what's mentioned above, god knows what Mandark could do to her in that defenseless state.
Aside from the Fridge Horror example in "Dexter vs Santa's Claws" where Dexter's determination to prove his theory, he also completely ignores the following inconsistencies with it:
If his dad *were* Santa, he'd have a pillow stuffed in his coat due to being too skinny, and his glasses would be a dead giveaway. The hairy stranger has no glasses, and does not appear to have anything crammed in his coat.
The family car must weigh a least 5 tons at the most, so not only could any person be unable to move such a heavy object without starting it up first, but it'd also most likely crash through the roof in a split second. Not that anyone could make it look enough like a sleigh that no one would think it was really a car anyway.
There were eight reindeer, so if Dexter's mom really *was* one, who would be the other seven? Besides, no reindeer costume could possibly look realistic enough to fool anyone, as in Dexter's rap, she's just wearing a light-up red nose and fake antlers.
Even if anyone could get a car safely on a roof without it falling through, as mentioned before, driving off of it could potentially injure and/or kill a person, no matter the circumstance.
But quite possibly the biggest inconsistency of them of all is Dexter doesn't think to look in his parents' bedroom to see whether they're there or not, as people can't be in two places at once.