I feel like Rosalina can win in every match-up in the game. Luma has top tier footsies, and even if you lose it, you don't get hit for it. I'm not really ready to say any character at all counters her, but most match-ups against other good characters seem competitive. She's really complex though so you have to learn a ton of stuff to play each match-up. I don't think I can pick out negative match-ups for her right now; I might have a better idea in 6 months?
I also disagree that her best match-ups are versus heavies. Heavies have many advantages against Rosalina; Luma can generally poorly protect itself from their moves that kill it so fast, and since Rosalina is primarily playing a zoning with normals game, their ability to practically kill off pokes and punish stuff with large grab range are both huge assets. With your main Charizard, I've run this match-up a few times not to my favor against Steeler, a pretty strong Charizard from Wichita. I think Rosalina can win it for sure and I need to just do better (feels evenish to me), but here are the issues. Flamethrower is an amazing slow paced neutral tool, and slow paced neutral is where Rosalina is strongest. Even if she blocks, you're hitting the Luma which is a kinda real problem for her; rely on this a lot to force her to step things up which is forcing her out of her comfort zone. If you get into a footsies war with Rosalina herself a bit closer, your jab is faster than hers and does more damage; any Charizard of course should be inclined to jab a lot since he has such an incredible jab, but this match-up especially highlights it (if Luma is synched, your jab will generally lose: beware!). If Rosalina's strategy is to jump at you, a very bad strat, Rock Smash it; she cannot outspace it, you armor her aerials, and it's all bad for her from there. If she comes from a more vertical direction, usmash beats everything and even if you don't have much time utilt does the same thing. Rosalina is going to be chipping away at you with clever footsies stuff especially when she has both characters in full swing on you; you as Charizard need to find opportunities to safely wear down Luma or catch Rosalina in a footsies battle with Luma either dead or too far away to help. Juggling Rosalina is effective for many characters due to her physics and Charizard does have a decent uair, but it's tricky with his limited mobility and her generally strong dair; I advise going for it some but being prudent and knowing your limits on it. Charging into ambiguous situations where either character can do stuff is probably going to end badly; definitely avoid that. Also, try to limit how much you jump at her either when she's on the ground or in air to air; she has all kinds of ways to deal with that though if you have really tight spacing you might be able to bair Luma to an early grave with relative safety. If customs are on both Luma Warp and Shooting Star Bit are hugely helpful to Rosalina in leveraging Luma to make your life hell, but Dragon Rush is also going to help you a lot in killing Luma and getting quick punishes on her as she tries to command Luma or retreat as she often needs to so I don't think it changes the match-up just makes it more exciting.
I know the Falcon match-up less well, from what I've played it feels more solidly in Rosalina's favor mostly because it's a lot harder for Falcon to kill (with Rosalina's physics, she's extremely hard to combo at kill percents, and Falcon is a lot easier to play safe against and extend stocks than Charizard if he's not getting kills in combo situations). I'm not going to pontificate on this match-up a whole lot since I haven't played nearly as strong of a Falcon as I've played a Charizard, but I'm just saying if you main Charizard match-up shouldn't be stopping you from beating a Rosalina at or below your own skill level.
