What a gorgeous game! The visual style is gorgeous and instantly drew me to Beyond the Underground - and it was on my list of jam games to play very early on. There is a very high level of polish here - from the start menu with an animated logo all the way to a nice credits page, it feels professional and well-made from start to finish.
The music is also lovely and matches the graphics really well! My only feedback here is that it could be a tiny bit more varied. Maybe a different track per level would mix things up a bit and show that the user is progressing?
For me personally, a couple of quality of life additions could be added. First, it was a bit jarring that the user needs to refresh the web page to reset the game - I think a "Restart" option after the credits would be a better choice. Also, I didn't know that you could continuously press the jump button to go up the bubble streams and lost a lot of time trying to make some jumps because of this. I think a pause menu with a controls screen would be a good idea here, or some dialogue about the bubbles that displays the first time the player approaches some.
Gameplay-wise, I think the addition of some obstacles would add a sense of progression to the game. Maybe in levels 2 and 3 there could be some pathways that are blocked off by doors - and the user could unlock these by exploring alternate pathways to find keys. Also, the character is working through some personal struggles in their mind - and it's great that you've used this jam to explore this type of theme - but (for me, at least) the resolution feels a little sudden. Perhaps a "boss" at the end of level 3 would make this breakthrough feel a little more earned. Defeating them could result in the "Welcome back, it's so good to see you" text being displayed. The boss could be something as simple as a darker version of the player, à la Shadow Link in the Zelda games or Badeline in Celeste - but I couldn't say what the game-play would be to defeat them 🤔
Overall, this is a fantastic little game and I know it's going to do really well. I hope you don't find this review too negative - I've actually played Beyond the Underground 4 times now and I just wanted to leave this feedback because I know this game could be on the same level as other great games that tackle mental health issues - such as the aforementioned Celeste. Well done on this excellent game and good luck!