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 would have known that Dexter had an 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.
Can you say, "Tear Jerker"?
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).
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!
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.