I'm not seeing much disagreement over ZeRo's placement of Marth which is odd to me. I've also heard whispers from top players of him being top tier and a lot people on this site upset over his "low" placement on the official tier list when it first came out.
Did I miss something? Because as far as I know he hasn't done anything to distinguish himself from the likes of Toon Link, Pikachu, Falcon, Lucario, etc. All solid, top 20ish characters that perform well at regionals, are capable of breaking into top 16 and occasionally top 8 at majors but can't do it consistently. Just look at some of the recent results from his best solo mains.
Mr. E: 13th at Collision XIV, 9th at Shine 2016, 5th at Endgame, 49th at SSC, 9th at KTAR Saga, 13th at EVO, 33rd at CEO, 9th at Apex, 7th at MomoCon, 17th at GOML, and 65th at Pound 2016.
Pugwest: 25th at Shine 2016, 17th at KTAR Saga, 17th at EVO, 49th at CEO, 13th at Apex, 9th at TGC7, and 17th at Pound 2016.
So a lot of losing before top 8 at regionals right when the real competition steps in, just barely missing bracket at majors, and a number of straight up bad placements at some of the most important tournaments of the year. The only somewhat impressive performances from arguably the two best solo Marth mains in the world were their 13th and 17th placements at EVO. Certainly nothing that screams hidden top tier or a shoe-in for top 15.
Marth also has False repping him but his best results such as his 7th place finish at CEO and his win at Midwest Mayhem 3 came primarily from Sheik.
Leo has been taking many of his locals with Marth and had a big win over Mr. R. But he also uses a lot of MK and Cloud in deep bracket runs so we don't really have a good idea of how he'd perform going all Marth. When he pulled him out against Ally and Larry he got 3-0'd by the former and immediately switched back to MK against the latter. We're also still yet to see how Mexico's players (other than Leo and Hyuga, far and away their #1 and 2) would stack up to the competition at American majors. If you ask me, Leo's just miles ahead of his competition. Even Dabuz said that he didn't have trouble with anyone at SF5 until Leo.
I dunno, I can't help but feel that popular opinion of Marth is riding a bit off of hype and confirmation bias at the moment. I could be wrong, who knows. Definitely still a solid character though.