Stupidity and validity being decided by whom? If you believe someone's opinion is stupid, you may not consider it valid, but that changes nothing about the actual opinion. Remember, Newton had a series of "stupid, invalid" opinions about physics and astronomy, and now look at us.

(rpmisms and hnbroseph both make excellent points! I want to add that "Stupid opinions are not valid." is, of course, just my own stupid opinion, eh? Combine that statement with "Opinions are valid, even if they're stupid." and you have a logical self-referential paradox, eh?)

perhaps it's a category error to apply the notion of 'validity' to opinions.