A dimbulb is a stupid person — someone whose mind does not “shine”, like a lamp whose bulb is too dim. It is informal American English, often written as two words, dim bulb, and it sounds a bit jokey, like a cartoon insult. You might hear it in old comedies or when someone is teasing, but it is still rude if you aim it at a person who did not ask for it. It is not a classroom word, and British speakers are less likely to use it.