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. -Phoenix 79
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. 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.
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. I'm more partial to the idea that Dee Dee basically became as broken as Dexter did after Mandark became the Executive of the Corporation. While yes, Dee Dee is the object of Mandark's obsession, it wouldn't change the fact that he is still the 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 get 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 Mandark's machine! 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 defenceless state.
When we see just how advanced Mandark's lab is in his debut episode and the ease with which Dee Dee completely trashed it, just think about how often she wrecked Dexter's lab and their corresponding attitudes. Mandark's lab starts out even more advanced than Dexter's but it's smashed almost beyond repair by Dee Dee whereas Dexter's lab gets regularly damaged but recovers quickly. Dexter's lab's been exposed to wanton destruction so often he's had to make it relatively modest but rugged whereas Mandark's had no real adversity until he showed Dee Dee his lab, which left it totally vulnerable to her active ignorance.
In "Dexter vs Santa's Claws" before the ending when he suddenly says something, why did Santa only say "Ho Ho Ho" to Dexter instead of using normal words to explain he is not Dexter's father dressed as him.
In the episode "Bad Cable Manners" everyone in the episode kept calling Dexter's father "Mr. Dexter's Dad" instead of his real name... how exactly does this work?