fltk looks promising but the support is lesser than on qt i would say...
I have seen some examples where people run Qt projects on rpi boards, even if they are very basic it looks easyier to accomplish than any thing else.
If my application is very small the Qt runtime framework should not be too heavy, is it too big to fit in ram or on the swap?
Maybe Qt is against the concept of tinybcore linux because of size and performance?