My god, honestly such a platform needs to happen. You just cannot release apps these days without strong, on-trend design, or you are going to drown. And the standards are just too high to try and learn yourself. I myself am plouging countless hours into Sketch, and am getting better, but have all but given up on ever getting the subtelties of, say, typography right.
I lurk a lot on /r/graphic_design and find the “fuck you, pay me” ethos is pretty strong over there. That’s intimidated me from approaching designers myself, like, at all. Which is a shame because I think it is just as equally implausible to expect a designer to learn to code up their own apps to the standard required. The synergies are there, with so many freelancers working in both pools.
TLDR: No such platform that I’m aware of. Separately, I’d like to PM you some time when I have my code up to scratch and see if you’d be interested collaborating on this one.