Speaking of Gamemaker, here's one of the first games I ever made using Gamemaker 7 like six years ago or something with one of their basic tutorials:
It's called "Car Warz 2" (yes, there was a Car Warz 1 too) and it's literally just a scrolling road with two tanks that shoot at you and respawn immediately at the top of the screen when they die, complete with sound effects from TF2 and CS:S. (It has the Gamemaker lite watermark because I had to recompile it with a newer version.)
Nowadays, I'm trying to make my own 2D game engine in C++ instead, and in my opinion, learning programming is much more fun than using proprietary software like Gamemaker or Clickteam Fusion unless you really just want to see results, and it will probably be a lot more useful for you in the future.
It's certainly possible to make games using apps like this (
Undertale, for example, was made in Gamemaker, and
Freedom Planet, my favorite 2D platformer of all time, was made in Multimedia Fusion 2) but the result won't usually be fantastic to work with. Especially in the case of Freedom Planet, where I've heard horror stories from the game developer about how terrible it is to add new features and fix bugs that crop up at random due to limitations in the engine (although the game wasn't exactly programmed fantastically to begin with). Needless to say, they've jumped ship to Unity (and C#) for the sequel.
Anyway,
@Moonrat, I personally think the artstyle of your game has a pretty charming appeal to it, despite being simplistic and looking kinda "MS Paint-y". Depending on what you're going for it might actually not need any drastic changes. But if you want some inspiration, here are some prototype screenshots from Freedom Planet 2 which kinda reminds me of your game because it has a similar resolution: