Pretty much this.
Often when someone says they don't think Zenos is a good villain the response is that they only like villains who have some level of moral gray to them or they need a villain to have a tragic backstory or something. That's not really the case though. One of my favourite villains of all time is Kefka. No real moral gray there and I suppose a tragic backstory that never really gets explored or explained beyond "it's why he's crazy." Heck, in FF14 alone we have Asahi/Fandaniel. Crazy, Evil. Imminently punchable and with basically no real redeeming qualities. But entertaining and interesting. That's what Zenos lacks. He's not entertaining, fun, interesting, etc. He's beyond even just being boring because he's been boring for so long now and, ideally, should have stayed dead after offing himself in the Garden. Nothing major that's really happened since then has even truly required it be Zenos in that role at all.
If you just want to have a dangerous threat of a character, you can do that too. FF14 just did it with Ranjit. The difference with Ranjit is that while he was mostly just a badass physical threat to the player/party he only stayed as long as he was needed/useful. If anything, it actually left you wanting to know a little more about him. When you're dealing with the pure physical threat type of villain, they by necessity have a very short shelf life. Otherwise they stop feeling dangerous and just start becoming a joke... like Zenos.
Another major issue with Zenos as he was presented was that the player had no real input in just rejecting him, shutting him up and putting him in his place. One of Zenos pillars was that the WoL was just like him, fighting and killing for sport, always looking for stronger opponents to test themselves and feel something. But when you're dealing with an indeterminate player character, that just doesn't work. Maybe your WoL may feel that way, sure, but mine doesn't. Mine, outside of game mechanic necessity, only fights because they need to and because they are the only one who can. They're not out to test their strength or find glory, and the portrayal of Zenos being some sort of mad mirror to my character just doesn't work. It doesn't make sense. My character wouldn't have given in to a stupid fist fight at the end of the universe unless, frankly, it was the only way to shut him up. Which is pretty much how I in-character defined what happened after telling Zenos he could chew rocks and keep thinking whatever he wanted about my motivations because he clearly wouldn't ever get it. Sadly, the game didn't give me the option to just leave his sorry butt there with no fight at all but at least it gave me the option to not agree to his stupidity.