Edit: So latest versions of Retroarch Wii U now integrate Picodrive!
It works fine, still Doom suffers from slowdown though..
There is currently no dynamic recompiler support in the Wii U build, so 32X (and maybe MD's Virtua Racing with the SVP chip?) have performance that falls a bit short of satisfying, as they must be fully software-emulated.
Running dynarecs on the Wii U is somewhat complicated, because memory can only be mapped as writable or executable, but not both--which is exactly how a dynarec works, writing and executing new code on the fly. To do a dynarec on Wii U, you have to toggle the dynarec's memory between writable and executable while the software is running, and the work to do that hasn't happened in RetroArch for Wii U in general or in PicoDrive specifically.
Like everything, it comes down to somebody with the interest, time and ability to step up and solve the problem, but full-speed 32X emulation is definitely theoretically possible on Wii U.