If it's "obviously not an answer", then, yes, flagging as "not an answer" is appropriate.
That said, if the post is clearly unsalvageable (i.e. it wouldn't be useful even as a comment), it may be better to flag it as "very low quality" instead. This has a couple of advantages compared to "not an answer":
It adds the answer to the Low Quality review queue, where it can be deleted by reviewers without ever getting to the mods.
Also, something that you might consider an advantage is that, if the answer is edited (presumably) to improve it, the VLQ flag is automatically removed without requiring moderator attention.
The major limitation of VLQ flags is that you can't flag an answer as very low quality if it has a positive score. (Of course, if it only has one upvote, you can just downvote it first and then flag it.) This is generally a good thing: if several users found the answer useful enough to upvote it, it's probably not completely worthless.
Ps. Even "not an answer" flags don't always have to involve the mods — users with 20k+ rep can vote to delete flagged answers (with a negative score) straight from the flag review page, and if three of them do so, the answer will get deleted before the mods even see it. Still, if you think an answer should be deleted (as opposed to being converted to a comment) and it hasn't been upvoted, VLQ is generally a better flag choice than NAA.