Can't say about Plasma Mobile though, but that's the point why investing effort into that would be useful. The base is more promising than Gnome one.
I can't find a good introduction on the topic at the moment, so I'll just quote Microsoft's succinct answer: "Alignment makes your UI look neat, organized, and balanced and can also be used to establish visual hierarchy and relationships."
They also have a nice illustration: https://docs.microsoft.com/en-us/windows/uwp/design/layout/a...
This kind of thing matters for usability. KDE has always been ahead in technical features, but GNOME has for a long time been way ahead in paying attention to basic principles of visual design.
Personally, I never had problems with KDE design. If anything, I prefer it to Gnome's. Plus KDE is so customizable, that if you don't like something, you can find a way to change that.