I genuinely feel that way. Dark stories in Mario RPGs were something I have missed SO much, what [SPOILER] did with those flowers was genuinely terrifying (but in a good way)
And the [SPOILER] section smack dab in the middle of the final dungeon was genuinely one of the coolest things I've ever seen in a Mario game
The Glohm was a good opposing force, it really felt like the antithesis to the togetherness and teamwork that lies at the heart of the story, and that made it feel like something truly worth fighting against
And then of course the whole Zokket twist, and the Glohm only being a stepping stone in [SPOILER]'s plan, really felt like something that would be in one of the RPGs from the "glory days" like Super Paper Mario
TLDR: I'm really glad Mario games aren't afraid to get dark anymore