There are two primary meanings to 'superlative':
- to an extreme degree
- to an extremely positive degree
(the first is more of a description of grammatical words, the second in more common use)
For the plain old extreme degree, the antonym, or opposite, would be to a minimal degree, of no distinction, and the antonym would be something like
(though the latter has the connotation of not just middling but bad).
Your context though is looking for words of an extremely negative degree. So 'superlatives' as a noun is the same as a superior quality to an extreme degree, whose opposite is an inferior quality to the same degree...and there's no exact word filling that description.
Though this is not the same situation, I feel bound to mention that the word
meaning 'there is none better' (which is somewhat in the same camp as 'superlative') has the accepted antonym meaning 'there is none worse':