Probably a decent chunk of the perceptual issue you're seeing comes from their choice of font rendering library, which is noticeably "the IMGUI font rendering library", and looks amateurish. This is stb_truetype, which is a very primitive library, but is easy to install and incorporate. The other library is freetype+harfbuzz, which produces higher quality output, faster, and with lower memory. The cost of freetype is that installation is absurdly difficult, the maintainer is offensive, and the code is ancient, unreadable, and architecturally a disaster.
I don't think the chosen GUI library causes the other UI issues you perceive. The author probably hasn't put much time into learning visual design, and thus fell into whatever the UI library supported. But to produce a better one, the author would have had to do a lot more learning and thinking, which is independent of the library choice.