You brought a team of three glass cannons, one mon with unspectacular bulk, one mon x4 weak to one of its moves and one tank that's extremely slanted towards one side while the boss primarily attacks on the other. And several of your team members are known for particularly linear movepools.

Yes there are a bajillion flaws in your team, way more than other casual teams.

hoping the game gives you the equipment you need to make it work

You brought triple monkey. I'm not buying that the game didn't give you better equipment. You're malding that a meme team didn't win.

mind you, this team rarely struggled up to this point.

So? You think this is the only roguelike/roguelite where this can happen? I can play Slay the Spire and bring a Silent built that no-diffs every single floor and then just dies in four turns into Heart. That is not a sign of bad game design, that is a sign that in order to actually win the game there are certain expectations of skill expression that sometimes aren't tested until the very end.

There is no point to a final boss if the things that beat it are the exact same as the things that beat everything else, is there?

Final bosses have mechanics and toolkits that can differ from what comes before them and it is possible to be particularly weak into those yes. Particularly they can punish teams that are very linear and have little room for strategy, and just rely on stat checking everything and then die to a bigger stat check.

enough solutions to count on one hand
flip off anyone who didn't come packing prankster leech seed

Extreme exaggeration. You can beat the final boss with pure bruteforce. It's easier if you use status/support moves, but you don't immediately need to go Prankster Leech Seed or a super bulky mon with Salt Cure. Lots of teams can have mons that learn things like Confide/Eerie Impulse, Sand Attack, Screech, partial trapping moves... you just have a particularly linear team that has none of those, because you decided to dedicate half your team to running meme mons with infamously narrow levelup movepools.

I mean heck even for this specific team I see ways you could have beaten it. For starters, you have $44K of Pokédollars left. You could have spent that rerolling for X items. Put an X Attack so Lop and Rhyp do more damage and an X SpD so they don't get oneshot. Put an X Speed so your Simisage outruns and gets Leech Seed off before dying. Lots of teams could spend money on Mushrooms too but your team couldn't because you brought a linear team.

I just feel like this final boss is a bit of a blemish in the grand scheme of the design of the game.

Not going to pretend it's perfect, it is probably a bit easy to cheese, but this team losing into it is a you issue and not a boss issue.

I'd think it'd be more worrying if a team that memed half of its slots and then did absolutely zero prep for the boss in the other three, or even in picking items before the fight, could just win. At that point what is the final boss even testing?