It may feels that way to you, but KDE, and especially Plasma (since Plasma 5) has been designed by professional designers. We owe this notably to Jens Reuterberg who created the Visual Design Group within KDE, a group that is still very much alive. The feeling probably rather stems from the fact that KDE’s vision for design is less inclined toward a strongly polished, opinionated interface, but rather to preserve user’s choice?