The only problem with the visuals was their simplicity.
If you wanted to improve this on your own, there's a couple of things you could look into depending on how you want to approach it.
If you want to do it yourself, I would recommend looking into pixel art. Small pixel sprites can be very forgiving to artistic skill, and when you figure it out, you can make the assets pretty quickly as well.
If you don't want to make the art yourself, you can look at using asset packs. There's loads of free ones out there, and as you do more you will figure out how to mix and match assets from different packs to get the look you want.
If you decide to go down the road of using assets, just be mindful of any license requirements. Personally I try to make all my own assets wherever possible. It means I own them and can do anything I want with them, but it is a lot more work and limits the scope of what I can normally make for a jam game.
If you want to see what I mean, have a look at the other games I've published here.