I think it's just general perception that makes people think Shulk is Top Tier.
You have a character who is indeed vastly improved from his Smash4 Incarnation. Even though it's not Top Tier, making Top 30 in Orion still paints Shulk as a competent character especially when his usage according to SSBWorld is rather average. He's an average character used with average to above average results. But more importantly because we know nobody actually pays attention to results that aren't massively major tournaments, we can guess most of this is just, general perception.
Shulk with Mondo Arts looks good, on paper and through data Shulk looks good. But this is the same thing people said with
and while a few people might put Ganondorf in High Tier, I do think the general consensus is, at best, Mid Tier. But I remember how hyped King K. Rool was when he was first introduced. Everyone thought this animal was the best heavy ever, like what madman gives a heavy such a recovery? And Super Armor? King K. Rool was the greatest thing ever made and you're dead great grandfather who served in World War I literally popped out of his grave just to tell you that King K. Rool was the best character in the game.
We all heard it and hell, you all probably believed it too.
It wasn't as bad as K. Rool but I remember when people gushed over how amazing Ganondorf was now. Not only was he killing 50% earlier but all of his aerials had been improved, he had new techs with his sword.
Now unlike those two, Shulk actually has some results to back himself up, but it the same thing. Shulk with all his improved Arts and techs made him "look" good. And in respect, a lot of that has made him good. I actually have no problem putting Shulk in High Tier. I'll also say I'll sooner put Shulk in Top Tier than Ganondorf in High Tier, that's for sure.
People say "Results" aren't all that matters, maybe not but it paints a pretty strong picture of the character. Plus with things like the SSBWorld Stat usage, we can now get a good idea of how much these character's are utilized.
Shulk is a very average, some might say slightly below average character used. However he makes a bit above average results. That tells me that despite having average popularity, the majority of his players bring him results. Which is something I can't say for other characters.
So while I question Shulk's ability as a top tier, unlike so many other character's who literally don't have jack to their name, Shulk at least has some results. Results that are superior to his apparent usage. So he's one of the few characters you might get away with saying
"He'd get better results if he was more popular"