Eh, I have moderate trouble against zelda. Fair/bair come in at nice pika-face height, and her fsmash has decent priority against pika's approaches. That being said, she's horribly immobile, decently easy to edge-guard, and easy to ko if you can pop in a jc usmash. Her big gun is her fair/bair, so if you watch out for that her game kinda deterioriates.
I find myself doing evenly against zeldas, but it always seems like i'm making stupid mistakes. It just looks to me like as long as the pika's smart, he's got a lot more to worry zelda than vice versa.
On the fox matchup, it's tough. Yes, pika does a lot better against him than a lot of the cast, but he's top-tier for a reason. Pika can chainthrow him, but any decent fox will put on enough pressure and hit his l-cancels enough that grabbing is going to be difficult. Also, on any stage other than FD, he can usually DI away to a platform, wall, or uneven land to tech and escape the next grab. On FD, you'll likely get him to about 70% (or off the stage, whichever come first). The thing is, he can do some decent damage to you if he grabs you as well, and it's probably easier for him to grab you than vice-versa. Grab leads to uthrow which leads to uair, which can then lead to another uair at lower percentages, and outright death at higher ones. His projectile beats yours, he can reflect yours (which, while slowing him down somewhat, doesn't prevent his movements that much, and can now hinder your approach with the reflected jolt), and he has a few more approach options. Dsmash is pretty good for tripping him up, but if you miss one you'll probably get usmashed. And, while his usmash is slightly less powerful than yours, it's still the second most powerful in the game, and will kill you pretty quick. Or lead to a uair for some nice damage-stacking.
Basically, pika does well, but Fox is top tier for a reason. Basically, pika spends most of the match waiting for an opportunity to gimp kill or abuse fox's fastfalliness, and fox spends most of the match chasing and otherwise beating the crap out of pika. Unlike other matches, you can't outrun or outmaneuver your opponent much in this fight waiting for an opportunity, so you have to play it fox's way. Which fox happens to be better than you at.
Not sure that made sense. Oh well. Zelda matchup is iffy, I'll wait for others to respond. Fox matchup is decently hard though. He's just too good.