For this critique I'm not going to talk about bugs. Everyone knows bugs are bad, I don't think bringing them up will help you improve as a dev unless they're particularly common or completely game breaking. Also for other people reading this, I personally know the developer of this game, which is why I might be a little harsh towards them.
First off, arguably the most important thing in a game is communication between developer. Communicating too little can prevent players from experiencing a game, and too little can strip player of a sense of discovery and potentially make them mad. You're game does both. This is my opinion and you're free to disagree, but I think the first message communicates too much. How to thaw the crystal is fine, but I think by telling players that they can mine ores gets rid of the opportunity to make players feel smart by hitting a crystal and seeing it break. This is mostly a nitpick, but I think there's something to learn from it.
It's how little you communicate that the game really suffers from. The main thing is having no response when hitting an enemy or crystal. I found it impossible to tell the attack range because the slimes behavior would show 0 change. A sound effect would've worked wonders to make the combat feel more natural
The visuals are the best part of the game in my opinion. I don't know how you got it so stylized. The only thing I want to critique is that the slimes look weird when you can see their polygons. I personally recommend just putting a subdivision surface modifier to increase their polys
Other than that the only complaint I have is that the movement is way too fast. Overall congrats on your first solo game jam and first 3d game. I know with time you can be a great dev