Hi, Villager main here. I'm still newer to Smashboards but here's my opinion on the Rosalina MU.
I don't see much discussion about the MU with Customs so I'll start there.
Pushy Lloid can push Luma away from Rosalina without blowing up, so try keeping Luma away from them. Ways for dealing with them could be GP, launching Luma to blow it up, or using Star Bits.
The Trip Sapling and Counter Tree are going to be really annoying as usual. Luma is immune to tripping so if you see him planting a Trip Sapling maybe send Luma in there. Watch out if he grows the tree, though. Luma can easily trigger a Counter Tree's attack, and the long lasting hitbox from a growing tree will likely hit Luma if you do send it in while Villager is growing the tree. Maybe Luma Warp could teleport past the tree and get Villager if he camps behind it.
Although Villager doesn't have much of a use for Pocketing in this MU, he can still use it for B-Reversing for landing mixups against Rosalina's juggling. So there's that...
2/3 of Rosalina's recoveries with Customs lack hitboxes, and that could make edge guarding easier for Villager. Edge guarding Villager is rather difficult especially with the Exploding Balloons, but it's not impossible.
In general, Rosalina's height makes her an easy target for slingshots, but her disjointed hitboxes and ability to deal with projectiles does make me think Rosalina has a slight advantage over Villager, so +1 for her.
Also,
I'm still new to competitive Smash. I started a couple months after SSB3DS came out. If I completely missed/overlooked something, said anything stupid, or anything else like that, please point it out.
Thanks and happy Smashing!
-AJL