It's not really a question of opinion. Stupider is, as the dictionaries and usage you quote show, entirely grammatical. It is also in rather common use, though less common than more stupid.

The main issue here is that words of more than one syllable tend to resist the -er suffix. Wiktionary's entry on the -er suffix says this (emphasis mine):

The suffixes -er and -est may be used to form the comparative and superlative of most adjectives and adverbs that have one syllable and some that have two syllables.

I believe that people's aversion to this word stems from the belief that "-er is usually used for monosyllabic words" is a rule, rather than a guideline (or, more correctly, an observation). This tendency probably has phonological roots related to stressed/unstressed final syllables interacting with the suffix, but I really don't know enough to say.

If a word has a meaning everyone can agree on, appears in dictionaries and is in use for centuries, this means it is a word!. It might be a word with social connotations of "improper speech" or "bad grammar", but a word nonetheless.