Fast characters give her the most trouble, especially short ones. Big characters are basically a free win.
Palu's biggest weakness is her frame data; when you have an 8f jab and no good OOS options, you don't win when someone gets on top of you. Nair OOS whiffs against most of the cast, fair OOS is good range but whiffs shorties (and is slower than a shieldgrab), while usmash OOS is usually too slow to punish with. If palu whiffs a move and someone gets in close, she's going to have a tough time getting them off of her. Fast characters have a much easier time getting in, forcing her to space fairs more as opposed to flinging projectiles or bair walling. Against some characters, this is enough, but any matchup that forces this play pattern is at best even.
Despite everything I just said, I don't think pichu is that bad. Maybe I just haven't fought a good one yet, but you can get off some serious punishes against him, culminating in very early kills, which offsets his size and speed. I agree that wario could be tough, mostly because of his massive air control, which gives him a huge amount of effective range, and also makes him difficult to catch with EF or uair for kills. I personally struggle vs some of the better projectile spammers, but I think that's because I don't use her reflector enough, so I don't feel qualified to comment on how those matchups go until I get more experience vs them.