Stupid is just a stronger version of silly. But generally when you call someone stupid is because they're doing something dumb or acting stupid or are unintelligent. Silly on the other hand is more playful. If your friend paints his face red and makes funny noises you might say he's being silly.

Yes, but "silly" is not as strong as "stupid", and can be used affectionately e.g. silly billy, silly sausage.