The Simpsons - The Why of Bart

if you think about it (and i refuse to think of 'the kids as all-fight' as canon on that), Bart never acted stupid until he went to school (and 'coincidentally', saw how Lisa did in kindergarten, with her smarts and pretentious attitude)
before then, he was mischievous, but not stupid (as the show often shows him to be, despite many many episodes that prove he's not stupid).
so it's either homer's fault (what with all the strangling and insulting (he literally never calls Bart by name, unless he's in trouble. it's always 'boy' or 'the boy') with his poor parenting skills, or he just lacks attention (as in, he's always in Lisa's shadow, and therefore wants others to notice him for a change. which they don't unless he acts like a menace)