You can edit some stuff by poking around the .ui files but a lot of stuff are not in the ui files.
For mockups, Inkscape is the best tool around if we are talking about FOSS. And about proprietary ones I have see a lot of UI/UX people using Adobe XD, but I haven’t used it, nor I know much about it.
My mockup was made in Qt itself where I just made a new widget do all the drawing and popups in the code which is of course C++. Note, we use the same stuff in Krita too. If you want to go that path, of course you can download QtCreator and experiment yourself, there is lot of stuff online.
If you want to hack on Krita itself, we can surely help. A couple of years back, a law student who didn’t have done much C++ stuff, contributed a patch, so it is not impossible. Take a look at this thread.