So first, I'm not all that familiar with second edition, so I don't know how they compare. But what I can say is if besides the campier monsters, like the Space Clowns and the Vampirates, there are also some darker and edgier creatures, like the Feyr.
"A feyr (pronounced “fear”) is a tentacled horror that feeds quietly on strong emotions. To minimize the chance of harm to itself, it prefers to devour the nightmares of other creatures while they sleep."
or the Neh-thalggu
"Known throughout the multiverse as brain collectors, neh-thalggu consume the brains of Humanoids and use them as receptacles to enhance their magical abilities."
So its not all Humanoid Penguins (Dohwar) and Giant Space Hamsters. They also released a supplement a while ago called Monstrous Compendium Vol 1: Spelljammer Creatures, which had things like the Astroid Spider, Eldritch Lich, and Puppeteer Parasite.
I plan on doing a full review of Boo's Astral Menagerie once I get around reading it in full, I've only skimmed it.
RE: Spelljammer: A Review of D&D IN SPACE!!!