What's the reason not to use Plasma Mobile though? I'd prefer investment of effort into that.
I see the same problems on desktop Plasma, wildly inconsistent padding, UI elements not lining up and bits of the interface crushed and cramped, while 100px below you have gigantic elements.
If you're going to take a swing at doing a FOSS phone right, you really need to avoid the wonky programmer art or you are not going to convince the average user that its any better or more polished than previous failed attempts.
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.