No one here is arguing that "evil is fun." That is such a blatant strawman argument that ignores/misrepresents what people are ACTUALLY saying here. "Serious simply for the sake of being serious" can be problematic, but so can "funny simply for the sake of being funny." The level of darkness or humor should fit the material. For example, Batman being pretty dark/serious overall works because, he's that kind of character. And being funny fits GOTG because the whole selling point is "a team of lovable misfits going on adventures."
This is Thor RAGNAROK. You know Ragnarok, the "end of all things," basically the Norse apocalypse where all the gods will die in battle. That is not a premise that screams "lets do a bunch of goofy humor/gags," it should be taken more seriously. That doesn't mean that there cannot be ANY humor, even TWS (which took itself pretty seriously overall) still had it's fair share of that. But the kind of humor that was in TDW, or GOTG, or the IM films? No, that doesn't fit this premise, imo.