If you act dumb are you dumb?

Yes, for acting dumb.

Acting 'dumb' doesn't mean you are 'dumb'. Acting means that you are portraying something, and portraying means that you are not the thing you are portraying, because otherwise, one would not need to portray one's self if one was already itself. In other words, acting dumb is different from being dumb, and acting dumb doesn't necessarily mean you are dumb.