So I waited a few days and tried to play this three different ways to make sure I wasn't giving a skewed opinion/first impressions. I tried this on my phone through the browser, My PC through the browser, and on my Odin 2 via the ROM download.
This is too hard. I noticed Odin 2 felt the best to control, but the game is a little too strict with how it wants you to jump. Most jumps are sandwiched between 2 obstacles in some way and the game is expecting you to be near dead-on with your timing. You also have a lot of difficult jumps in a single room. That combined with only having 3 hit points and a full on room restart for a single mistake is rather annoying. The player sprite's hitbox is also a little ambigious, it made timing jumps correctly off platforms even more difficult.
I was able to get better at it eventually, I made it out of the first run with 3 mice after multiple restarts. But it didn't feel rewarding, it's nowhere near enough to buy anything from the shop, and I kinda don't wanna continue to find more mice, I've seen the other areas and they don't get any easier. I understand you're shooting for a difficult experience, but it comes across feeling more unfair than anything.
If you could tweak the physics, or the hazard placements to be a bit more forgiving it would work wonders. You could also give us more hits instead of three, or make mistakes more forgiving with better respawn points. But I really think the core problem is the physics. If jumping was a little bit easier to control the other issues would vanish. But I know it's a balancing act with this sort of thing.
Just wanted to say I'm aware this isn't the final state of the game, and programming platformer physics is not easy (idk if it's even a thing you can really do in GB Studio). But, I am also a fan. I own the other 2 Batty Zabella games and recently spent time 100%ing the first one (yes even the secret spider area). I clicked on this the minute it popped up in my notifications. I think everything else about the game is perfect, music graphics all that Jazz. I know you make good stuff and I look forward to seeing it when it's finished.
Hope you find my critiques helpful, and good luck!