I don't think it is (almost) ever appropriate to post an answer as a comment.
Since this might discourage other people from writing a similar answer that can be voted on and possible accepted
(for other reasons, see
If the comment is just a partial answer posted as a pointer for others, then it should stay as a comment.
As for comment sized answers consisting of just a link:
1) If the link is to a website that can be trusted to remain available and static (e.g., Mathematica documentation, another stackexchange site, a particular version of a Wikipedia page) then a simple and polite comment sized
This question is addressed in this part of the documentation link
is probably acceptable as an answer. Although I would not feel comfortable accepting rep for posting just a link and would probably make it a community wiki. This also encourages other people to expand on the answer if they want to.
On the other hand, it does not take much effort to post extra comments or code samples along with the link, so that should be encouraged.
2) If the link is to a website that can not be relied on to remain static, then the relevant content should probably be moved into the answer and the link given as a reference.
Finally, if there is a comment that you think answers the question well, there are a couple of things you can do about it. (Taken from https://meta.stackexchange.com/a/90270/156389)
- Write a comment that requests the comment to be posted as an answer. (Be sure to ping the person using @UserName)
- If that gets no response in a reasonable amount of time, then copy (and maybe expand) the comment as an answer. If you don't feel comfortable getting rep for it, then set it as a community wiki.