I've just read this quotation here at StackExchange: "Think of how stupid the average person is, and realize half of them are stupider than that." I've checked a few online dictionaries and there seems to be nothing wrong with the sentence. However, if we say "more acrid than" and "more valid than"; why not say "more stupid than" ?
stupid -> stupider -> stupidest
is equivalent to
stupid -> more stupid -> most stupid
You can choose which version you would like, they mean the same thing.
The rule is typically that if an adjective is monosyllabic, you add -er to the end to make it comparative, but use the adverb more for a polysyllabic adjective. There are cases that flout the rule, however, mostly through common usage. They can be represented either way.