I'm curious what you would consider to be a better UI kit. Personally I think Material Design is the smoothest and most professional looking thing out there. Everything else seems too cartoonish or just overall less appealing to me.
Qt and GTK I could see being seen as 'cartoonish' because the default themes on many distributions are fat, shiny, bubbly, and look like they firmly belong in the 2000s.
Cocoa, maybe because of the vivid blue? Or that person hasn't seen Cocoa since before they moved away from the glowing blue bubbles, although I think they still look nicer than Material.