I've been working my way slowly through the mermaid tag, and I've seen a lot of mermaids tagged topless, nude, and often both.
We discourage the use of both tags for the same person in general, because otherwise a nude person would gain dozens of partial nudity tags. However, what do we do for characters who don't really have a "bottom" to be bottomless?
Personally, I recommend that unclothed mermaids be restricted to nude. There are a few instances of mermaids who are topless but wearing a sarong -- e.g. The girl on the left in post #446334. -- so the two tags aren't purely equivalent. (Side note: I wouldn't recommend tagging bottomless on most mermaids & lamia with unclothed tails since that's essentially the default.)
While I'm tagging and doing general cleanup on mermaid, I don't mind going back and straightening this out, if we can get a consensus. What do you all think?