1. Although it doesn’t look or sound right, “stupider” is the standard comparative form of “stupid”:

2. “Dumb” does mean “unable to speak,” but it also is a near-perfect synonym of “stupid.” (However, “stupid” does not share the definition of being unable to speak; its primary meaning is “unintelligent.”)

Keep in mind that “dumb” and “stupid” are often considered as offensive and informal. Words such as “inane,” “obtuse,” and “vacuous” are less offensive and also provide subtle nuances that “dumb” and “stupid” do not. (This is yet another reason why having a good vocabulary helps.)