Well, that's the tricky part. Especially because it's so hard to get people on the same page about the series' aesthetics in general.
For monsters as characters, I think the Zeti are overall a move in the right direction; they're clearly set apart from "normal people" but there's a similar philosophy in terms of color and the stylization of their proportions as there is with the main cast, so they don't end up looking completely out of place like, say, the Black Arms or Iblis spawn. They could maybe stand to have a little more in common with how the main characters are designed, or at least be more coherent with each other; different body types and silhouettes are typically good character design but the Zeti are kind of all over, you've got little pill body Zik, super lanky Zazz and his long face, Zeena's proportions being a little too human, and whatever the hell is up with Zavok's whole mouth area...there's kind of a fight between the details that tie them together as a species and the ones meant to make them each look unique. I guess the Babylon Guardian from Riders wasn't too bad either, at least as a monster based off of the Babylonian bird people compared to a monster meant to be inhuman.
For monsters as basic enemies I think a lot of the Dark Gaia spawn were decent. Similarly stylized/exaggerated proportions, color choices that feel similarly vibrant as the main cast's but are unique to them and fairly consistent across them, but more menacingly and inhumanly built, like the Nightmares being hunched over with big toothy jaws and spikes, the Titans being big spiky brutes with just one big eye, and the Big Mothers having stomach mouths to barf enemies at you.
And for giant monsters...honestly it's hard to think of one that I feel really fit the series. Perfect Chaos isn't too bad, but kinda pushing the limits of what I'd like to see. I guess the Nega Mother Wisp is ok? Not that I particularly like its design but it's kinda on the right wavelength. Mostly just take what works for the above categories and amp it up. Bigger, nastier, but still stylized in a way somewhat reminiscent of the main characters.