Good Topic!
http://www.smashboards.com/showthread.php?t=81784
Phanna made a thread a long while ago talkin about how certain characters can't actually be chaingrabbed by the ICs. Peach is one of them. Peach is floaty enough that she gets hit up too high to truly be regrabbed by Popo. So your job is just timing it so that u interrupt the regrab as soon as you are out of hitstun.
If you DI away from the ICs, you are forcing you trajectory forward, giving u less height and putting you right in front of popo's grab box.
Now I'm not terribly knowledgable about this, but my guess would be- C stick smash DI up, while with the control stick you should DI up and behind the ICs. According to phanna, peach can jump out if its timed. But peach's jump is super slow so unless ur gonna air dodge immediately, I'm not sure how good it is to simply jump out. Peach's jump brings her slightly down before going up, which gives popo extra time for the regrab. So perhaps an immediate nair would be more effective than jumping.
Again, I don't know from experience, I'm purely speaking in theory what sounds like it should work best.