I fully understand that opinion - it is a big pain point for many users. Inside KDE, our VDG (Visual Design Group) is working on ironing out those inconsistencies and visual problems. It's a gigantic task, but I do believe we are making progress - especially System Settings has been getting a lot better lately. They are also working on a evolutionary update to our default 'Breeze' design theme. Check it out at https://phabricator.kde.org/T10891.

(And if you do find specific problems e.g. regarding margins or fonts, please report them on bugs.kde.org, and we'll take a look.)