peach does very well against the fox and falco in my experience. Her defensive play style and quick, bulky hit boxed sex kicks coupled with her FCs really help shut down their offense and lead directly into her comboes and tech chases. She pressures extremely well on sheilds, and gets great reward for doing so, just like they do. her turnips help out a lot at preventing approaches and forcing reactions, which really helps escpecially with fox when it comes to his speed. with the addition of AGTing, her ability to manage her own position while still covering space is even better than it was in melee. when it comes to killing, peach is gonna try to edge guard all day, while the spacies have their usual bread and butter kill set ups. peach is good at working them offstage through tech chases with dash attack and her floats. once they are offstage, she can easily chase them farther off then they can even survive from, and her nair, bair, fair, dair, and toad counter all can all usually take the stock depending on how far and where they got knocked offstage. if the peach plays it right, she can garner kills at lower percents than she can even be killed at. when it comes to damage building, both sides have good combos and projectiles. the space animals are quicker, and their projectiles are much more conductive to damage building than peaches are. peach, if she can land a grab (which she doesnt have too much of a problem doing), has a chain grab on them with uthrow, which can lead to a kill at the end of it with upsmash or a nair/bair into tech chase.
Overall, they all combo well, have great pressure options, and have potential to kill efficiently. i think peach might be slightly favored against falco, just due to his lack of speed compared to fox, and peach garnering a couple buffs like range increase on ftilt, increased utility in her fsmash, AGTing, and her more defensive utilt. with falco not being touched that much from melee, id say their once even MU might be put slightly towards peach, mostly because of her increased position control with AGT. fox id say its more around even, mayble slightly in foxes favor since he kills a little bit more efficiently and reliably than falco does, and his ground mobility is better.
Peaches worst MUs are going to come from characters that can really take advantage of her poor speed and bait her into playing offensively, then punish her. CF, Jiggs, wario, ROB, ZSS, pit, and wolf all have attributes that give them the options of not needing to commit to approaches, being able to cover themselves from below, and all have the mobility in the air to take on peach. There are much more specific things to each of those MUs that could make a couple of them more close to even possibly, but overall, those characters have the attributes to take advantage of peaches biggest weakness.