Nausicaa brought it up once, but pika will actually have more trouble with solid, constant hitbox characters rather than rush down, quick conversion characters like spacies.
In a way, spacies are limited to the same sort of precision play that pika has to play. Then we have a character like peach who like melee, does incredibly well against pikachu. Why?
Well we can again look at what pikachu has and what peach has that counters it. I brought up pikas ability to close gaps quickly while being able to preserve many of his options until the second he strikes. This limits fox and what he's able to do. If fox wants to keep his dd game going, he must sacrifice the ability to throw out his anti air utilt. On the flip side, he has to sacrifice some of his mobility if he wants to have utilt available to throw out on the spot. Becomes a complicated mixup of baits and committing to approaches and blah blah they are stuck playing the same game.
Then we have peach, who arguably is one of the best characters when it comes to preserving her mixups and her ambiguity. She does this with a mix of tight dd and float shenanigans. Unlike fox, she can cancel her dd by immediately double jump landing and now she suddenly has all of her options available. So we have a character who preserves a mixup of basically her entire moveset regardless of if she's moving. Now pikachu has to worry about everything peach has to offer.
Then looking at peachs moves we see that they have high priority and often linger. They're also very quick to start up and lack any real indication of what is coming. We can't really know on reaction if peach is going to fc a bair or empty land into immediate dash attack. (This is actually a huge asset that a lot of people don't even actively consider, in addition to her having some of the only frame tight, completely safe shield pressure) Finally, aside from dash attack and dsmash, her moves are difficult to punish. Pikachu, who excels at finding holes, doesnt really have much of a hole to work with. His aerials will be beat out in general and it begins forcing him to use more committal moves. Pikachu doesn't want to have to use usmash in neutral, but peach can force it out of him because it's one of his few moves with enough priority to contend with peach.
For pikachu, it becomes a game of calling out bad floats and dd camping.
We also have pika losing a lot of his edgeguarding potency and although his recovery is still great, it matters less because peach doesn't rely it. She is happy to play a battle of attrition and force as many 50 50s as possible. She's going to beat pika in the long term battle almost every time. She simply likes trades more and puts out more damage in general.
So what can pika do? Well first he has to stay grounded as much as possible. A good peach knows that pika doesn't really have much to threaten peach with in the air. She can basically cc option select everything. Doesn't matter if pika tomahawks, dsmash is coming out during his landing regardless. Now this was way worse in melee and has been improved a bit in pm since pika at least has fair and the occasional bair to contend cc with, but he's still very limited.
The mu boils down to peach staying safe and not pushing too hard. She needs to be a rock and slowly choke the rat out. Pika is stuck playing really lame. He doesn't have lasers to force approaches or anything like that, but he also doesn't have the tools to really threaten peach up close. He's kind of stuck with a mixup between 2 or 3 options in any given situation, all of which are usually pretty linear. It doesn't matter if pika approaches with shffl nair, usmash, dsmash, or grab, peach's design gives her a universal answer. She just cc option selects into dsmash. Aerial? Cc then punish? Getting close and no hitbox yet? Just start the dsmash. Not like she can't just cc again if she was baited.
So now pika is playing a character who can safely counter pikas approach, doesn't get early gimped or edgeguarded, doesn't care that she herself is missing edgeguards, and wants him to rush in for easy trades. That is a very uphill battle. He has things like soft nair to usmash and early thunder kills, but getting that opening in the first place is tough to do. Not to mention how mentally draining it can be to play the mu. It's hard working your ass off as pika for a minute, controlling the pace and reading your opponent, just to get dsmashed once and have percentage be even again.
It's not unwinnable by any means and pikas new dsmash and fair really help the mu, but it will always come down to the fact that the risk reward is heavily skewed in peach's favor. She just benefits more from the messy neutral that happens during every match since we're constantly adapting to each other.
This is me trying to discuss productive things
Edit: lol I just saw the nausicaa comment after I finished this post, that's pretty funny. I think I'm a little better at speaking the language of smashboards then he is, and I usually refrain from posts like this until I've had it very thoroughly thought out.
Personally he's one of my favorite posters. In fact a lot of what I've said sort of aligns with his views too, regardless of who is better at articulating it in an organized and simple manner (he can definitely be a bit all over the place). I don't always agree and sometimes it's hard to follow, but there are a lot of somewhat abstract-ish comments he lays out that line up with my own thoughts. I think one of the biggest turn offs though is sometimes reading and figuring out what the mad man is trying to say seems like more work than it's worth. I definitely commend the amount of effort he puts in though and he always has a unique perspective.
I often disagree with posts but gain a new avenue of how to think about the game just by reading how they think.
I think it's really cool reading people's thoughts and trying to analyze how they came to certain conclusions, and what they may not be considering which you feel is important to something you're talking about.