You're asking the wrong question. Everyone plays each character differently, so while a top player or even two or three top players playing Pikachu may have negative matchup odds against a certain character because of how another group of top players play, that by no means implies that you'll have a bad matchup against that character too.
So how can you find your good/bad matchups then? You need to find them for yourself. And the secret to that is...
Matchup experience is simply how much experience you have playing against a certain character. It's how much you know about their combo starters, how they like to approach, how their moves work, etc... And in gaining matchup experience for a slew of characters, you should start to get a feel for which you have trouble countering.
But how can you gain matchup experience? There are two ways that I've found. The first is to...
Get more games against more characters
Play against a wide variety of good players playing a diverse group of characters. I'd suggest making some friends in your local scene. If you don't know your local scene or how to find it, here's a comprehensive list of Smash Facebook pages around the world. If you have a scene with more than 15-20 people, your city is probably on there. You could go to their tournaments and then make friends to play with on the weekends, but if you're a little shy or don't have a smash scene, you can also enter in any of a number of online tournaments.
The only one I've had friends participate in is top US player, Nairo's, free bi-weekly tournaments he calls "Naifu Wars" that usually garner upwards of 500 entrants. But if you're interested in online tourneys, you can find a whole plethora of them over on smashboards.
There's also the online mode within the game itself, but I've found that I learn more about myself and other players when I'm in tournament since every game I'm trying harder than if I was just playing online.
The other way to gain matchup experience would be to...
Learn from the pros
While a character may not be your hardest matchup like it is the pros', their difficult matchups can indeed show you how you may be punished by certain characters. For more on this, I'd strongly suggest watching ESAM's matches to see how he plays. He's currently the #1 Pikachu in the US.
I personally find the difference between these two methods to be like the difference between reading about math theory vs. doing math problems. While reading theory or watching videos might give me a broader idea, the only way I'm really going to understand and learn is by trying things myself. So I'd strongly suggest getting out to some tournaments (whether online or physical) and meeting some new training buddies in the process!
An aside on matchups...
It's also worth remembering that this game is only two months old as of this Thursday. Characters that people think are good right now will almost certainly not be the same as a month or two from now. For a comparison, you could look at a history of Melee Tier Lists. Even though the game was released almost 18 years ago, matchups that are supposed to be "unwinnable" or "easy wins" are still changing even today!!
What I'm trying to say is that information you may find from what other people are saying or even from what you feel while playing may not always stay true. The advice I've given above however, will likely always be a way for you to feel who your problem characters are and who you have an easy time with. Just make sure to keep playing and stay active in the scene!