Anyone who hasn't put themselves through Brawl ICs impact of the meta is usually someone I don't see as truly understanding what the **** is going on. Putting Sheik above Diddy plainly ignores his match up skews,
There are top players claiming that Sheik is better than Diddy, top players who have experienced the Brawl ICs metagame. And it's also surprisingly short-sighted of you to say that you think that someone needs to have played a
previous game to understand what's going on in the current metagame.
Blaming yourself is fine, but there are limits to your character, if Pika has game breaking abilities to overcome this, then you wouldn't be switching to Diddy or Luigi.
A 50:50 (or heck, 55:45 or 45:55) matchup with Diddy does not imply that Pikachu will snap the game in half and erase Diddy from the SD card when selected. It implies simply that when two players are playing equally well (note, emphasis, red markings, etc.), both have about equal chances of winning.
Pikachu gets punished for mistakes much more than Diddy or Luigi in the Diddy matchup.
If I'm not playing perfectly, but I'm still better than my opponent by enough to win with a lesser-played character, then switching to Diddy or Luigi is the logical choice.
Why juggle tons of different options with subtle differences in their usability, DI d-throw properly 100% of the time, mix up Quick Attack, remain constantly mobile, edgeguard consistently offstage, etc. when I can pick Diddy/Luigi instead and just grab my opponent when he tries to grab me? It's far simpler. It doesn't mean that the matchup ratios are more or less skewed. It just means that it's easier for the player to demonstrate skill when he might not be playing at his best.
However, I have admitted that I'm finding that Diddy has more tools in the matchup than he had in the past, and that's why I've dropped my ratio from 55:45 to 50:50. Pikachu has some great tools in this MU as well, including a resistance to SH f-air, a ground game comparable to Diddy's own but better because of Quick Attack, better mobility, better recovery/edgeguarding/offstage, a slightly better camping game, and a much better platform game due to QAC.
Pikachu hasn't been shown to do a single thing super-noteworthy bar a Day 1 WiiU tournament.
Losing your best representative is a pretty huge blow. I daresay that Pit and Dark Pit would be like, lower Mid Tier without Nairo, if not worse than that. Robin? Poor Robin might be bottom 5.
If you take the strength of Pika's chaining/combos and look at how his tools don't allow him to maintain a dominant positional advantage (unlike Diddy, Sheik, Falcon, Sonic, Rosalina) afterwards, then the pressure the player has dealing with Pikachu diminishes drastically ["oh this could happen, but it's not that debilitating to me or my game plan at all"].
Being in range of Pikachu's down-tilt is an obnoxiously bad scenario for almost every character in the game. Roll behind, get hit by up-tilt. Roll/move back, get hit by Quick Attack. Jump, and get punished because you have to hit your f-air low to the ground to hit Pikachu at all. There is counterplay for this, but the advantage is obviously in Pikachu's court when he's in that sweetspot range. And offstage, Pikachu is obviously at advantage.
I'm not going to continue this discussion for the reason that I'm sure that people are tired of the Pikachu discussion by now. Customs are here and people want to hear about other characters, not the same Pikachu debate that's been rehashed time and time again now.
To finish I will say though that as a person I have no character loyalty. If I think that Pikachu isn't offering me anything valuable in tournament, I will be the first to drop him/forget about him. I play to win. I don't play Pikachu because I like Pikachu. I play Pikachu because I believe that in a customs-off metagame he's top 3 and has a better matchup spread than just about every character. The moment I stop believing that he offers me any value (which might be true in customs), he will disappear from my roster, simple as that.