I really like the octopus eyes and eyelashes! They're cute, and the lashes strengthen the characterization of the octopus beautician.
For further development, I'd like to see clearer definition on the mirror and the mermaid's right hand.
The mermaid's face is highly stylized, but I think it would be more pleasing and likeable if her expression was not so exaggeratedly thrilled. I would relax her mouth slightly so that the sides/corners of her upper teeth are not exposed. To take it to the next level, I would also try to refine her gaze so that her eyes are focused just right on the mirror. It's a difficult thing to draw/paint a gaze precisely, but using a webcam on a timer to capture photos of yourself looking at something in the exact same position relative to your gaze as the mirror is to the mermaid's could help you dial it in.
I think the concept and general composition are both good. I look forward to seeing you complete this piece!