I was watching the updates for this one on the MGGJ discord server!
The graphics and audio are what stand out the most to me. It's got a really charming look and feel that I like a lot.
I'm not sure how much of the SHMUP I'm feeling it in currently. I mean, you shoot bubbles, so fair enough... but without dodging or y-axis movement, it doesn't feel very SHMUP-y to me. In fact, adding Y-Axis movement would probably be my biggest suggestion. Alternately, if you want the character to be constrained to one location (like a platformer) maybe add the ability to jump to dodge things?
There's not a ton of content currently, but with more added I could see this being a really fun game!. Great work!