I doubt most Guitar Hero charts would make for good Mania maps, in my opinion. Especially the ones with fast strumming or anchoring.
EDIT: Also, it's a pain in the ass to properly time maps in this game -- which is probably the most important part, since osu!mania depends on hit accuracy, unlike GH/RB. Given that most rock/metal songs fluctuate slightly in tempo (load up in an editor and you'll see all the slight BPM changes), it might be frustrating for people to map them. I'm mapping Metropolis Pt. 1 by Dream Theater and I have probably over 100 timing points by now, maybe more, even though there's really only 3 extremely noticeable tempo changes, for example.