Uncle Ted is Derek from the future
Look how similar they are in appearance, especially compared to the rest of the family. When Derek and Bobby grow up and become adults, Derek regrets how mean and abusive he was to Bobby when they were kids. So, using a time machine and a memory-altering device (both of which were invented by adult Bobby, see the super-genius theory below), he goes back in time and poses as Bobby's fun uncle. He used the memory-altering device to make everyone think that he's Martha's bother. He hopes to give child!Bobby as much fun and good times as possible, to make up for child!Derek's abuse. Bobby grows up to be a super-genius
His big head holds an oversized brain. Because he doesn't really do any schoolwork yet, his mental powers come out through vivid imaginings. Howie Mandel has cartoon world powers.
He discovered these by learning how to use the voice of Bobby, which summoned Bobby from the cartoon world which Howie is originally from. Howie can switch between Cartoon Dad and himself at any time even in the real world (see "The Visit to Aunt Ruth"), and uses this to become Bobby's dad in the cartoon world.- Seems to be true.
Derek is so mean to Bobby because he's jealous of him.
Since Bobby is little, he doesn't seem to have many responsibilities, while his older brother and sister Derek and Kelly do. That, and once Bobby was born, he 'stole' Derek's spot as the baby of the family, which would probably result in sibling jealousy, especially since Derek might've been so used to getting what he wanted by then.