I actually wrote a long post about how I felt about the designs yesterday and deleted them, specifically in response to this:
@Wagomu:
People keep saying this aaaaaand… what show does it look like exactly? I mean there's inspirations here and there, like the facial expressions being reminiscent of Flapjack, or some art design looking OK K.O.-inspired, but there really isn't another show that looks quite like it. That's nothing compared to the old shows where the characters either looked like they were based on the same toy molds everyone else used in the 80s or looked like an anime-inspired take based on the designs based on the same toy molds everyone used in the 80s (admittedly, 2011 Panthar, Snarf and Mumm Ra aren't bad, but the rest are kinda generic).
The character designs are like a concentration of what some people call the "calarts style" and I just call the popular trend in animation of today. There's nothing new about shows and artists sharing an overall animation style. As has been mentioned, anime do it a lot and the old Hanna Barbara cartoons almost seemed like they came from a factory, but this is the first show with this specific style that seems to lack some charm about it that makes it unique compared to its peers. The peers I have in mind are Gumball, Clarence, Steven Universe, Gravity Falls, Star vs., etc.
I like the art in most of those shows, because there's something that separates them and makes them unique, but Thundercats Roar looks to me like if someone plugged Thundercats Go into a template. The thick eyebrows, round eyes, bean shaped head and the smile are all there. The proportions are all the same, the thin lines are the same, the chibi-like body proportions. All of the hallmarks of the style are there, as they are in most of the shows above, but there's nothing that makes it stand out, like Steven Universes video-game inspired settings, Clarences pure nostalgic comfiness, or Gumball's multiple art methods and styles interestingly packed into one show.
I don't think it looks bad by any means, but when i first saw it, its adherence to the style was the first thing I noticed. I guess in summation it just doesn't stand out to me, and it stands out so little that it kind of stands out for not standing out.