I had a friend beating me as capt falcon pretty consistently and I'd say he's pretty good, or at least, captain falcon is the only character he's competitively good at. I got frustrated and just did a 100 stock against a lvl 9 CPU captain falcon to practice pblocking falcons b moves, specifically down b and side b, which my friend was using to get rid of luma easily. Those moves can be easily punished after blocking. The down b punish can be kind of tricky depending on where he starts his kick. If he starts the kick close to you the as soon as he goes through you immediately turn around and do a dash attack. If he starts farther away, I find that up smash usually works since he'll end up more or a less on top of you and up smash can cover both forward and behind. If the up smash ins't working i'd suggest a side tilt if he ends up behind you or a grab if he ends up in front of you. These are suggestions for when falcon is in control.
Once you have wrested control from the falcon he becomes much easier to deal with. I usually just force him to a ledge and start throwing him over the edge. If you land your forward air while captain falcon is below the axis of the main platform a Fair will gimp him every time no matter the percentage. At higher percents (even like 50%+) just landing a forward air anywhere off the platform, as long as it's not super high up, will usually gimp him. Since his recover options are pretty limited it's actually really easy to hit him with a Fair in most cases, especially if you're the one throwing him off the edge. Of course, throwing luma off the edge can even further limit his options.
When I'm fighting a character like this who seemingly counters rosalina, I just try to keep the character off the platform at all times so they have very few opportunities to actually use their ground game to take rosalina out. The characters I most often use this tactic on are: marth/lucina, little mac(lol), ganondorf, captian falcon, and bowser. I'm sure that there are more that it would be useful with, but I don't know a whole lot of matchups just yet.
For characters that have superior air moves, or just a really good recovery that makes fighting off the edge like this a lot more risky, I tend to just let luma do the work. I've had the most success sending luma out over the platform to bait out a jump or recovery move from the opponent and then easily punishing it from the safety of the ledge.
Anyway, hope this is helpful, Captain Falcon is actually very easy to deal with once you know the matchup.
Rosalina4lyfe