True, but just add -ly and it's grammatically correct. "It fits badly." Although, interestingly, "badly" as a post-verb adverb tends to denote, not the opposite of "well," but the strength or measure of something that is already understood to be bad. We would say, for example, "It hurts badly." "Badly" here just means "a lot, and therefore bad." "It fits badly" is grammatically correct, but doesn't seem to make sense semantically--why is it bad that it fits?
But I agree with KarenKaise2, "poorly" is the proper word to use (to mean opposite of "well"). It's strange, but true.