First of all, I want to say how much I love this and how much I want to draw a punk mermaid now. I love the idea because pretty, elegant mermaids have been done to death, and here you come along with exactly what I wanted to see! She's funny, oozing with attitude and I've felt that sentiment many times. It's a gorgeous concept, and I love the little details you gave her, right down to the tears in her fin.
There's two main things that bother me with this- the anatomy and the folds of her shirt. We'll start with her anatomy. Her human half is fine, a fold on her elbow would be nice where the fat and muscle rest on a bend, but other than that I have no real complaints. My real bugbear is with her tail- it could be argued that mermaids don't exist so noone knows what a mermaid tail looks like, so I'll use the little mermaid as a comparison. If you look how ariels tail sits, it has a definite bone structure underneath, supported with muscle, fat, and finally skin. It makes the tail feel more solid and has some real strength and weight to it. She could happily swim in a sea with currents and water resistance whilst still looking delicate and stylised.
Your mermaid tail doesn't seem to have that, and seems to wiggle about fairly loosely with no real construction to it. (between the hip and the fin). If she was dropped in the water, she wouldn't get very far as the tapering off part wiggles all over, and therefore wouldn't have any strength. Also, it makes your drawing seem a little thoughtless and careless, especially when your human half is looks so lovely.
I suggest, as well as drawing as though there's real meat and bones in there, you study what's called the line of action, and get your mermaid tails to follow it, as it'll give you a much stronger pose.
My second problem is the folds of her shirt. The creases seem very random to me and look like it hasn't been ironed, rather than creasing as her body moves beneath it. Creases are a hugely complicated thing to go into right now, so I suggest you learn about them from a GOOD tutorial or even better, from a trusted anatomy book.
Keep up the fantastic work!!!