Sadly, there is only one Smartian left: Megamind. The others were killed when a black hole devoured their planet. If this sole survivor is representative, Smartians are thin, blue, and have large, bald heads. They dress in latex and rubber and claim to love AC/DC although they are often heard listening to Minnie Riperton.
Again, it's difficult to extrapolate from a set of one, but it's easy to see why people dislike this particular Smartian. His appearance is shocking. Despite landing on earth as an infant, he never mastered English. He's egotistical even though his plans rarely work. And he's constantly kidnapping reporter Roxanne Ritchi.
Probably the main reason people dislike him is that he killed Metro Man, enslaved Metro City (which he mispronounces as "meTRAWcity"), got bored, and created a supervillain even worse than himself.
That being said, Megamind has a lot of good qualities. He keeps going no matter what (see: plans never work). He's a brilliant inventor (he even developed an invisibility gun so effective he lost his car for weeks). And he inspires deep loyalty in his best friend Minion (a fish in a robot gorilla suit). Even Metro Man is fond of him.
In the end, Megamind the Smartian redeems himself. He keeps Metro Man's fake-death secret so he can retire in peace. He defeats the villain he created and frees Metro City. And he wins Roxanne's affection.
Megamind wasn't always a villain; he became a villain because he was disliked for being different. Not completely human, but not good-looking like Metro Man. His speech just a little off. Always somewhere in between smart and clumsy. People don't like things that are different, especially things that are only half different. This is why the Jews didn't like the Jewish/Gentile
Samaritans.