It feels to me some feedback was a little raw here. I agree on that it should be not sold on Steam with using nothing but stock assets and the level design could be better for sure but in case someone is new to game dev and don't know how to make own assets and have no experience with designing levels, I would say it makes more sense to focus on content, gameplay and story instead of trying to make custom assets that going to look half that good as the stock ones and hang up on trying to make things look realistic with 0 experience.
I would say just keep going, continue to use stock assets you can also buy more, nothing wrong with using them as long you are considering them as placeholders only. Work on the story, gameplay and levels, can watch some youtube video tutorials about level design practices and once you feel your game is complete in terms of gameplay, story and levels then you wan to look in to creating some custom assets to make your game unique. You can start first by creating your own custom textures and reskin the models have been using, then you might want to look in to add your custom features to stock models, bring them in to 3D modelling tools and tweak them, finally look in to making your own 3D models from scratch.
I need to agree on that, do not share it in the current state but not because it is bad and you did not put in to any effort. Some might say 3 weeks only but that is not important. The real question is, how many hours did you put in to this. It was vacation time in most schools so you may have just put in to this game the same amount of hours as some people do in 6 months after work and school and family. So don't let those comments put you off. But it is also true, the level design should be improved and you really need to add some custom look to your game before you choose to share it publickly in a store.
So what I recommend is:
1. complete the story, levels, gameplay and it is ok to use stock and store assets for now.
2. improve the levels, take some level design tutorials, learn from them, learn from other games..etc and improve your levels
3. add your own touch by reskin the models and UI in the game, take some tutorials on how to make textures and how to change the textures of the models using tools like Gimp and Blender for example.
4. add your own touch by customizing models, bring them in to a 3D modelling tools like Blender and tweak them, you can find some beginner tutorials how to use Blender 2.8.
5. create your own custom models with animations and textures from scratch, you can take some advaned tutorials and courses on 3D modelling and animation in Blender for example.
Good luck.