Marco is likeable. His culture/ethnicity doesn't play a major role in the series, in fact other than nachos/tacos and being forced to say "libro" by Ferguson, his culture/heritage/ethnicity hasn't been referenced much. The only major thing was the Blood Moon Ball where Marco dresses up in a traditional Mexican suit + sombrero + skull mask, which is fine since he later infiltrates hell itself with it.
Moreover, Marco is modeled from Daron's husband, so it's understandable why Daron would use an idealized version of his husband as a prominent character for her series.
Same goes for Janna. We didn't even know she was "likely" filipino until the book, and assumed she was this crazy gypsy girl.
In fact, I believe out of all the currently existing MUH LORE shows, I feel like this show is the least preachy of it all regarding "diversity"/"progressivism" and all those other hyperboles. Sure, there are gays kissing, but their in the background, not drawing attention itself, and just a normal within the context of one the episode, saying "LOOK HOW PROGRESSIVE WE ARE!" The fact that there were apparently even more of them in the background than what people realized (Mabel x Pacifica expy) serves how well they handled this.