The light-blue highlight on Jack and Zero is a really interesting choice. It's not super saturated but still stands out on top of the rest of Jack's palette, and it's almost covering half of his whole outline. It makes me think of the airbrushing used for the live-action Casper posters, or the GBA Castlevanias giving the main character a super-bright outline for clarity. It works mostly well enough in this mockup, but I could see some headaches forming if Jack stands in front of more of those rock walls with their own highlights. You'd have to be careful juggling background colours in general, in a hypothetical full game.

The hearts aren't a big problem in my opinion (a lot of videogames have UI elements or terminology like collectibles that clash against most other thematic elements present), but I do think the visible Jack bust and close-up-examination panel take up a lot of real estate. If this is a platformer (and for all I know it's designed to 'play' like a beat-em-up with a floor to walk around on) then you need to allow for enough camera space so the player doesn't feel constricted and unable to see ahead/above/below to judge where they can jump to.

 The hearts look out of place. I think they should be evil looking heart shaped pumpkins.