The adjectives thinnish and baldish bring to mind dozens of other adjectives made with the same suffix: oldish, youngish, mannish, girlish, fattish, longish, yellowish, etc. But dispeptic-lookingish is the author's creation aimed at a humorous effect, and, at the same time, proving beyond doubt that the suffix -ish is a live and active one.
The same is well illustrated by the following popular statement: "/ don't like Sunday evenings: I feel so Mondayish". (Mondayish is certainly a nonce-word.)
One should not confuse the productivity of affixes with their frequency of occurrence. There are quite a number of high-frequency affixes which, nevertheless, are no longer used in word-derivation (e. g. the adjective-forming native suffixes -ful, -ly; the adjective-forming suffixes of Latin origin -ant, -ent, -al which are quite frequent).
Some Productive Affixes
-er, -ing, -ness, -ism1 (materialism), -ist1 (impressionist), -ance
-y, -ish, -ed (learned), -able, -less
-ize/-ise (realise), -ate
un- (unhappy), re- (reconstruct), dis- (disappoint)
Note. Examples are given only for the affixes which are not listed in the tables at p. 82 and p. 83.