Is asking the OP to consider upvoting an idea in Salesforce Idea Exchange (provided the idea is relevant to the feature the OP is looking for) an acceptable comment? For example, see comment in this question.
From my personal user experience, I've never thought of it as a problem.
I would say, yes, it is acceptable within a comment, or maybe even as an aside as part of an actual answer.
My gut reaction is that it would not make an appropriate stand alone answer (but I might could be persuaded otherwise with good arguments).