Yours is not an uncommon opinion, and I think that's unfortunate. I totally get it not being your thing, and that's fine.
But my mind one of the real strengths of Spelljammer is it's refusal to take itself too seriously. It's a setting that recognizes the inherent goofiness of D&D, embraces it, and doubles down on it.
It's possible to run horrific and serious Spelljammer campaigns - I have done so myself, and some of the last supplements produced for the setting leaned more in that direction. But to me Spelljammer overall emphansizes the sense of wonder and amazement and possibility of D&D, without the kind of jaded cynicism that led to stuff like Planescape and Dark Sun. Planescape itself is more or else a 90s style gritty reboot of Spelljammer anyway, in a sense.
To each their own. But I don't see how a hippo dude is any sillier than Slight Elf Variation #2387 (this time, with even pointier ears!) or cat people or turtle people or pick your staple.
It's all make believe, people.