Mega Banette has an actual viable niche in Gen 9 National Dex; granted, it's very cheesy.
Mega Banette has the ability Prankster, meaning all its Status moves go first. Banette also has access to support moves Destiny Bond, Will-O-Wisp, and Encore.
You may see what I'm getting at here.
Prankster Destiny Bond allows Mega Banette to trade itself for a terrifying setup sweeper; for example, Quiver Dance Volcarona. If the Volcarona decides to Quiver Dance again, simply Encore it. If it is a physical attacker, like Swords Dance Iron Valiant, you can use Will-O-Wisp. Not to mention Mega Banette's ludicrous 165 Attack means its Poltergeists and even Shadow Sneaks hit incredibly hard, putting pressure on these Pokemon. A lot of teams do not have multiple Pokemon that can sweep an entire team, and they have to be cautious to remove Banette before attempting the sweep. For example, a team may have Dragon Dance Dragapult and Iron Defense Body Press Zamazenta as sweepers, but let's say the opposing team has a Curse Dondozo, who easily takes on IDBP Zamazenta, alongside Mega Banette. Banette can then remove the Dragapult should they attempt to sweep with Dragon Dance, and if Dragapult switches, the switchin is getting hit hard with a base 165 Attack Poltergeist.
Although this may sound incredibly annoying, there is counterplay. Pursuit users like Mega Tyranitar and Kingambit are immune to Mega Banette's Prankster shenanigans thanks to their Dark typing, and they can easily remove Mega Banette (granted, they have to sacrifice themselves as well, but that can greatly benefit another sweeper). A good player will often trade Mega Banette for a wallbreaker that they do not need anymore so their sweeper can have an easily path to clean.
TLDR: Mega Banette's Prankster ability along with its whole host of disrupting status moves makes it one of the biggest nuisances to offensive teams. It is able to systematically trade with, or worse, neutralize an offensive threat, allowing the rest of the team to have an easier time walling the opposing Pokemon.
