Yes I agree. I almost feel bad for disagreeing with someone who obviously put so much love in his games and rightfully stands behind them, but I cannot shake off the feeling that there is a huge potential for improvement there that does not involve hiring additional people, more expensive artists, etc.
Just investing some of his own time reading up on the fundamentals of art, visual language, perception, color theory, could vastly improve his judgement. I try to build games for fun and learning, and I consciously avoid anything that goes beyond my (non-existing) knowledge about creating 'professional' game art. In my case I resort to abstract graphics or introduce very strict voluntary contraints (like: only 8x8 sprites), ie: similar to what games like Baba is You or boxboxboy are doing. If I were to ever shoot for a game that required 'professional game art' I would definitely invest time to get better at it before even considering to start the project.
It's a bit like the programming side of things. Don't try to build your own engine for your game if you barely know any programming, for example.