hmm...there is a lot of depth in this matchup, so ill split it into the 3 main phases: neutral, follow-up, and recovery phase.
1. neutral
Captain falcon: he has the second fastest running animation, so he is faster than mac, and his grab is ridiculously strong. his aerials give him mobility and versatility, which will keep mac on his toes. in short, falcon can attack from the ground or the air in multiple ways, and can force mac into uncomfortable situations.
Little mac: while falcon is faster than mac, it isn't by much, and is negligible, since his is the 3rd fastest. macs tilts come out ridiculously fast, and his jab or tilts can interrupt falcons grab if thrown out soon enough. macs smashes also are dangerous for falcon, as they can super armor through his attacks, as they are telegraphed (especially some of his aerials), so falcon has to play less aggressively than he is used to, since he can get punished for it. macs grab is very bad however, so falcon playing defensively by using shield isn't a hard transition for him.
verdict: both characters rely HEAVILY on the neutral game, their characters focus, speed, and design reflect this. falcon has advantage with aerial attacks and grabs, but mac has the advantage in terms of super armor and faster ground moves. in my opinion, the characters themselves are dead even in the neutral game, its the playstyles of the players and how skilled each is that will determine the victor in the neutral phase.
2. follow-up
Falcon: i must say that falcon has better follow-ups than mac by far, and mac is easy to juggle. his throws can lead to almost all of his aerials, and his d-air can even string into itself if the player doesn't tech. while i don't know a ton about all his combos, his throw combos are his bread and butter, and there is little stopping him when he gets his string or combo started.
Mac: sad to say, but there isn't a lot going for mac here, since he isn't a combo character in general. there are some important things to consider though. falcon is a fast faller, which means you can combo your dtilt together at low percents, and follow it with a ftilt, his dthrow also creates strings with his fair, and he can use his nair to break combo strings. counter can be used as hard reads as well to discourage be juggled. utilt is useful as well, and can string with up-b. there is potential, although its not a vast amount.
verdict: little mac has a clear disadvantage here, since falcon is just better at combos than mac is. mac has to make his follow-ups count when he sees them, and try his best to not leave the ground. but its still going to be difficult to avoid. falcon just has to keep doing what he's doing, and doesn't have to change his playstyle in his follow-up phase that much.
3. recovery phase- im going to put this out there, this is a nightmare for both sides.
falcon: while his recovery is better than macs (not by much though) he has an issue changing up his approaches. his recoveries are easily telegraphed, and macs d-smash is scary good against his side-b and up-b. on the other hand, he can easily gimp mac when he has the stage. his dair, nair, fair, hell all of his aerials are effective means to get the job done and his utilt can force it as well. falcon is in a candy store making up his mind on which way to humiliate mac when hes gimping him. as long as he mixes up his recovery, he shouldn't have to much of a problem. better yet, don't go off the stage at all.
mac: so...yeah...hes my main, but he has a difficult time getting back. the one thing going for him is that if there is a wall, it improves his recovery, and opens up numerous recovery options. but he only has three main approaches: go low and up-b, go high and side-b, or counter and pray he takes the bait. however he can gimp falcon relatively easily, as d-smash covers his low approach most of the time, and up smash covers any high recoveries. he can even gimp with a bair on his up b if timed correctly, or go for a dair, nair, or fair, depending on the situation, and it will do the trick.
verdict: PTSD. if either falcon or mac get knocked off the stage, its a scramble to get back, as both throw out an approach and hope for the best. falcon has a few more options, but mac has super armor to protect him from offensive approaches that falcon throws out.
i could go much more into it, each characters move set, play styles used for each characters, common tactics and follow-ups, gimping methods, frame data and how they clash, and so on. this is more of a general summary and analysis. ill go more into certain topics on it later on.
55/45 falcon/mac. this is how i feel it is in competitive play, and how it turns out for me more or less. buts its almost dead even.