I really like the concept and interpretation of the theme. And while simple, the gameplay itself just works well and is fun.
I definitely wish that there was more benefit to switching between dimensions, mostly because that mechanic seems the most interesting part of this project to me personally. I get and like the idea that you take and deal more or less damage based on which dimension you're in, but as is, I found myself mainly just staying in the monster dimension the whole time. It would be great to have more reason to switch back and forth while playing; maybe something to think about if you plan on expanding this.
I imagine with some more time, this could be really cool. And as is, it's an excellent jam entry. Great work!