Carl is very out-of-character and unlikable here because he constantly abuses and blames Jimmy for accidents that Carl obviously caused himself rather than accepting his own mistake.
Every time Jimmy saves Carl from accidents that he causes, he never thanks Jimmy once, and continues to insult him.
What a huge Karma Houdini he is. Carl never apologized for his mean-spirited attitude towards Jimmy, and didn't got punished for his behavior, so his behavior is rewarded.
Misleading Title: The title of the episode is misleading. When you read the title, you might think that Carl becomes an actual genius like Jimmy similar to the SpongeBob episode "Patrick SmartPants" or the episode of this series, "Sheen's Brain" from Season 2, but that's not even remotely close to what happens at all.
Sheen does not let Jimmy explain the truth about Carl to Elke, which is very annoying and repetitive.
Sheen was more annoying than usual, which would become his character in the spin-off show, Planet Sheen.
Elke likes Carl for being a nerd, which doesn't add up seeing as Jimmy is a nerd too.
The episode gives off a bad moral that living a lie is okay as long as your loved one has been lying too.
Some funny scenes like Atilla the Hun loving fudge brownies with nuts, and Principal Willoughby singing "His name was Rico, he was a show boy!"
In spite of Carl Wheezer being a rude and ungrateful jerk to Jimmy, some might see this as Carl trying to get even with Jimmy after all the times he was used as a Guinea pig.