I think it looks great but its style clashes with the tiles and the other character's style.
Guess you mean problem with perspective, but not the problem with style. I think both characters and tiles are in one style, so if we remove the armor from the succubus, then there will be the same body as that of a mermaid. But yes, there is a problem with mermaid's perspective, I agree with this.
Her arm and body become shorter by a pixel for some of the frames, it looks off. It's the sort of idle animation you'd see in an 8-bit RPG with tiny sprites, it feels out of place with a more realistic style. The arm also seems to get a pixel thinner in one of the frames.
I think it would look better to keep the body animation sub-pixel and subtle unless you have a particular movement for her to do.
I see, I will try to fix it. And yes, that was what I was guided by - old zelda-like games and other old 8-16 bit games. I think this is quite acceptable for a game that looks like a game from 16-bit console. Many Zelda-like games continued to use such idle animation on 16-bit and more bit consoles. It's something like a nostalgic tradition, so it seems to me that such an approach is quite appropriate.
I guess you were aiming a breathing animation, but now she is breathing very heavily. As she looks like some kind of mermaid or naga, I would suggest a horizontal sway instead of the vertical bounce for an idle animation.
Ok, thanks for feedback, I will try to fix it such way. And you're right, for such a creature swinging in the horisontal direction is also appropriate.
General problem for me that the sprite is too small, therefore shoulder/chest swinging is too intense while she breathing. I know that people on Pixelation like to advice example from "Chrono Trigger" in such cases, but I still don't understand how to use this. So now I have a good reason to try it again.