Hello and welcome to Pixelation!
Looks like you have a great grasp of clean pixelling, very nice.
Few things I see:
Main issue I see is wonky perspective...
Yellow shows mid-lines of forms, magenta lines show axes (as far as I can see them). You want axes being generally parallel (in a neutral pose) in orthogonal view, or converging to vanishing point in perspective. Mid-lines should be off-centred from the middle of the form based upon the incident angle of the viewer to the primary spatial axes (wow, sentence!). I know that sounded like weird abstract babble... even to me... what I mean is, if your view angle is straight-on to the side platformer-style, then your view angle is at 90 degrees to the plane the monster is on, so the midlines cannot be seen at all (they are sitting right on the outline). The fact you have midlines showing (ignoring the tail, since its wavering around in is own direction) means either the monster is not alined to the plane that the game is set on, or your view angle is not meant to be 90 degrees. But looking at the spines on the body of the monster, we cannot see the far spines (since there two rows of them in the reference), which implies a 90 degree view angle.
SO! It's a mixed perspectve! Wow that was long winded. Either you need to reorient the neck and head so their midlines aren't showing, or you need to reorient the body to match a kind of 3/4 platformer perspective. I have done a kind of mix of both in my edit below.
Next thing is contrast or pop. Feels like there could be more brights in there to keep the sprite out of midtones territory.
Actually I think that's all the points! Here is my (very rough) edit:
