I don't know what to say about the art! I've drawn dumb cartoons since small times so when someone says to me "hey can you draw this", I can generally crank something cartoony out in a few minutes if I'm not too bothered about the quality -- perfect for a game jam.
Lately I've been trying to develop my "real art" skills by learning semi-realism digital painting techniques. So, these fast and loose cartoons are a nice break from that...
There's not much to the process. If I don't immediately get some sort of mental image from the suggestion, I'll do a Google image search to get some references to look at. I work at a large size (like 2000x2000 or higher) on my iPad Pro (using the Procreate app and an Apple Pencil) and do thick black and white line art. I add flat color (and maybe some very simply shadows and highlights) on a layer below that. I reduce the opacity of the line art layer so it's not fully black, but a darker version of the color underneath. So basic cartoony cel-shading-y looking stuff. I export as PNG and they're added to the game and resized as necessary.
I felt like the boombox should look like it came from the Mushroom Kingdom rather than real-life, so it has music note speakers inspired by the music note blocks in Mario games. The line-art kind of reminded me of a pig for some reason, so I colored it pink. (??)
Unless I'm forgetting something, this is the first time my art has appeared in a video game, so it's really weird and fun to see it come to life, even though they're just front-facing, perspective-defying flats. I think I would enjoy someday creating art for a game that I was able to spend a bit more time on. I'd love to do a whole game of my own from code on up, but I can never seem to come up with a viable story / scenario to build on.
(PS: I've also been helping make pixely character sprites for another jam game, World of Blanks. Sprites and animation are definitely not my forté but it has been a good learning exercise.)