A couple a things that came across playing against a REALLY good Little Mac (no customs):
- Little Mac can REALLY easily catch Rosa's landing. Him mixing up either shielding, dash attack, forward B and just powering through with superarmor seems to beat ANY option you pick to land. Platforms even this out a lot, just beware of Usmash.
- Neutral is really damn hard. If a Mac shields a nair or other close moves, there's basically nothing you can do and the Mac can punish quite easily.
- Mac's down smash is a crazy edgeguarding tool that's not hard to hit either. Mixing up recovering high and aiming for platforms is REALLY important.
- He kills Luma in 2 hits.
- Edgeguarding can be really hard unless you predict a counter and bait it out. Dtilt tends to cover his recovery pretty good though.
- Killing Mac can be insanely hard: if the Mac plays defensively, you basically can't hit him and if he also has KO punch, forget about doing smashes. Mac can punish every whiffed smash really well, even without KO punch.
- Being close to Mac is both bad and good. You are sure to hit through superarmor because Rosa and Luma will both connect, but Mac shines in close up combat because of sheer framedata (and of course RosaLuma don't have good CQC besides Luma jab).
- Boost grabbing makes it easier to grab a Mac running away.
I'd say this is heavily in favour of Little Mac at low and mid level; -1 or -2 for Rosa
but then has a sharp raise toward RosaLuma at mid-high because of heavy edgeguarding and Little Mac's reliance on few tools, and it gets easier to see through his (excellent) footsies. Well placed spotdodges can mess up Mac quite well too. Platform play makes up a big part of this matchup.
It can go from 0 to +2 this way, evening it out to ±0.
For improving the matchup, try to be baiting really heavily. Fading back or crossing up with Nair just outside Mac's range (or with Lunar Landing) can work in Neutral, alongside safe Bairs. Don't get impatient for not hitting him - sooner or later the Mac will try to read the approach, meaning your bait will have worked: empty hop shield grab, empty hop dtilt/jab, sometimes star bits or Luma shot are great against whiffs. Do note that Mac's "'active" range is very big because of his dash speed. This can be evened out a little by aiming for platforms.
For ground game, a running Little Mac can be hit with dtilt or ftilt, and a higher forward B is not too hard to cover with Usmash (especially with Luma around). Dtilt is quick enough to punish with after you baited out a move, and from up close jab is best - you can mix up multijab for damage with jab 2 to dair or usmash for kills/setting up an edgeguard - beware of counters. Please do note that Mac's ground game is far superior, and Macs are generally great at baiting with footsies or quick moves like dtilt or utilt. His smashes have suprisingly little lag.
It can become hard for Macs when you mix up aerial movement with grounded movement a lot. Extended dashdancing is almost a necessity for quick ground baits, when mixed up well with turnaround star bits and platform play.
If you thought you were a patient Rosalina and Luma main, think again. Every move you do will have more purpose as to baiting than to actually hit - this is ****ing frustrating. Platforms are your friends.