As detailed elsewhere (see How to answer proof-verification questions?) I do not consider it as best practice to post only "this is correct" as an answer. Usually, something more can be said to make for an actual post (see the above mentioned post).
Dually, I agree with the view expressed on Best way of asking "check my proof" questions, that usually only asking "is this correct" is not a best practice, but instead a particular concern should be formulated. If somebody cannot articulate any reason why they think their solution might be incorrect, there is not really a case for asking to begin with. (This concern can be vague and fuzzy, but still some reason ought to exist.)
As a combination of the above two I think that to comment: "This is correct. Why did you think it might be incorrect?" can sometimes be a good approach to get to a more meaning full Q&A pair.
Coming back to the specific question asked, I think it is a judgment call. But in certain cases, in particular early on in the history of a post, I do think it can be justified to turn n answer that only says "This is correct." into a comment, and you can flag in that sense. Yet, I would advise against doing this in old cases where already everything is resolved, with accepted answer etc.