Whedon was the better writer but I think Carter had more respect for the crafting of a TV series he just liked bilking his audience for as much money as possible. Or maybe that was Fox as well. Either way both of them disrespected the fans, ie the very reason they had hit shows to begin with.
btw I think its a bit ironic that Whedon's downfall was ultimately triggered by his behavior during the Justice League reshoots when he started his career as a script doctor.
Regarding The Mayor just being a Big Snake, that wasn't the payoff of the S2 hints, The Mayor being revealed as the master villain building a town for demons to feed on was the payoff for the S2 hints.
His endgame being lame? Well, all the Buffyverse villains had pretty prosaic motivations when it comes right down to it; end the world by intent or incidentally. At least The Mayor had a personality.
With Whedon his personal downfall was that (though his ex wifes revelations about the affairs were the start) but in terms of his actual writing ability, that had already taken a hit long before the revelations about him as a person.
The Whedon quips is a known trope even for those who've not seen Buffy
Age of Ultron was a failure, even more highlited by the Russos Winter Soldier (best Marvel film of all of them) and Civil War (essentially an Avengers film in all but name) Films before and after.
The leak of his Wonder Woman script, which was derided across the board.
The ideas he had for Firefly Season 2 including Inara's Virgina where like WHUUTT which had people thinking yeah maybe its good it was cancelled.
Dollhouse which I found boring and creepy was cancelled , while as for The Nevers, uh I saw the trailers and a clip online and that made me not bother.
I always thought Tim Minear (who worked on Xfiles) was his best writer and basically made Angel not Joss.
As for the Mayor even at the time of Season 3 I remember there were criticisms about how little was expanded upon and that Harry Groener peformance compensated for it. That and Faiths fall which was felt against covered by the Actress and her chemistry with Groener.
Iv actually seen an interview with Harry Groener where he says for Graduation Day that Joss was still writing the script for those 2 episodes while they were shooting and that he'd literally come out his office with new script pages that he'd give to them. That gives the impression of very rushed last minute.
That leaves the comparison with Carter who had problems but they felt like the opposite reasons as Whedons