Do -ist words (e.g. racist, sexist, etc.) have superlative or comparative forms?
Some -ist words are used as adjectives, as I describe in my answer to "Adjectival Usage of Racist".
The -ist words that are used as adjectives, such as racist and sexist, form periphrastic superlatives and comparatives using most and more. This is in accordance with the general rule that restricts the inflectional superlative and comparative suffixes -est and -er to monosyllabic bases. (That rule is a simplification: -est and -er can actually be used on some bases of more than one syllable, and sometimes most and more can be used with monosyllables. But it's difficult to give a complete description of the distribution of -est and -er vs. most and more, and there is some variability in usage between speakers.)
So "most racist" and "more racist" exist, but *"racistest" and *"racister" don't exist.