I think that Shulk is top 5, yes. I don't think that most here agree, though, "OP" seems to be synonymous with "really really strong." I'll admit I've been defaulting to Pikachu in tournament because Pikachu is just so good, but sometimes I use Shulk and am wowed by how amazing he is, and the untapped potential. There's so much that people just don't know about Shulk's strategies.
For instance, when Shulk is at high percents, some people think that you can just time out Shulk's Smash Monado until he loses it, and then he can't kill you with a throw any more. This is false, because the moment that he's out of Smash, he can go into Shield and survive for a few seconds until Smash regenerates (he can also camp you out with Speed, but that's a less viable tactic imo since Shulk is tall). Vs. Ness, when you and Ness are at high percents, Shulk enters a little grab war in Smash Monado mode. If his Smash Monado runs out, he switches to shield, survives Ness' b-throw if Ness can land it, and then goes back to Smash once he's back on the stage and repeats. He can turn many situations into 50-50s just by doing this, mitigating the best strengths of other characters. I have very rarely actually had my important Monado Arts timed out because there's always a substitute art that buys me time in every situation until I get the art that I need back. It's quite useful.
That's just the tip of the iceberg, though.
I also see Shulks not having committed their arts to muscle memory, and going into an art while an opponent just respawned on a platform (you can cycle through the arts until the guy gets off the platform). This is very suboptimal; Shulk with full mastery of his arts is much stronger than Shulk with a fishing pole looking for his arts. Once Shulks figure out the optimal art patterns for each matchup and perfect their tech, I think that Shulk will be an easy top tier.
on the subject of Pikachu, yes, I don't see how he's not better than Diddy. Objectively speaking, it just seems obvious to me. It's an easy comparison and Diddy doesn't come out ahead in anything except having a massive f-air vs. sword characters, but Pikachu can just gimp many sword characters anyway, so that's not that big a deal. If nothing else, though, winning the head-to-head matchup is huge.