I am a bit confused on this, since "-ish" is a a derivational morpheme for forming adjectives meaning "somewhat Adjective". Based on my knowledge, a comparative is made by the suffix "-er" and not "-ish". Any thoughts?
This really comes down to how you define derivation versus inflection. The line between these two categories tends to be incredibly fuzzy and difficult to determine. For instance, if your definition is that derivation changes the category of a word, and inflection doesn't, then how do you handle participles (inflected forms of verbs that act like adjectives)?
There are problems with most of the nice clean definitions of inflection and derivation, and the best that I've seen is "derivation makes native speakers consider it a different lexeme". In this case, I'd say that "longish" is a fundamentally different lexeme from "long", so I would consider it derivational. But others might disagree.
So the best answer I can give is, "the dichotomy between inflection and derivation isn't as clean as intro ling professors make it seem, and isn't particularly useful in practice".