You seem essentially to be asking whether huh? can be considered a question. Well, that depends of course on what you mean by 'question'. In one way, yes, it is, because the aim of the speaker is to elicit a response from the listener. Mind you, a speaker could do the same thing without any tag at all (It's bad?). However, the tag huh is not what we teachers normally consider a question tag when we're teaching grammar.
Be careful not to confuse grammatical structure with pragmatics. In grammar, the idea of a question is associated with a particular order of words whereas in pragmatics it's about what the speaker means to do.