Piracy in One Piece isn't just about being a Criminal or operating at Sea
It's about completely refusing the shackles of being Part of a Kingdom which allows anyone to be welcome to raise that Flag
It's basically a Faith that People can unite through, instead of being Separated by Borders & Families & Origin & Race ... etc
So Joy Boy opposed Kingdoms & welcomes Outcasts, similar to Luffy & Blackbeard & WB & Kaidou ... etc
Joy Boy can only be considered as Successful as Yonko if He manages to create a Country of Pirates
(Every Yonko share this Goal, They just seek it in different ways & with different Intentions)
However, it's said that Joy Boy is from Ancient Kingdom, meaning He didn't Create it himself, and it was further Confirmed in Last Chapter when Emet said that Joy Boy didn't become "King", there are also other Major Kingdoms that predates Joy Boy's Time so not that either, and only Allies We saw so far (Emet & Zunisha & Poseidon) don't seem to have any Place to go to
Actually, Joy Boy apologized to Poseidon for not being able to provide a Place for her outside Fishmen-Island, and Zunisha not only predates Joy Boy but it's also suggested that Mink Tribe were living in Isolation for a Thousand Years with no indication that They ever found a New Place for them during this Period
Only Island that can be associated with Joy Boy is possibly Pirate Island, He might be it's Founder, but the problem is that it's more like Bigger Jaya, Wang Zhi was it's Ruler before & yet wasn't considered a Yonko showing that it doesn't suffise by itself, and all Pirates seem to be welcome there, regardless of who They follow
But Joy Boy clearly & definitely still managed to inspire People to become Pirates, suggesting that instead of creating a Country, He at least made People not afraid to fight back against the Kingdoms that mistreated them or kicked them out
In my Opinion, when it comes to Piracy, Joy Boy wasn't as Successful as Yonko, i think He was like Ace/Sabo Type of Character.
He was Ultimate Inspirational Figure but didn't Score that High on Paper, so better than Mihawk but lower than Yonko imo