In the first episode, it's revealed that Snarf is now a robot.

Lion-O is the biggest offender, as he is now a complete idiot, without any kind of charm.

The characters are depicted as dumb goofs who often act happy which little to other emotions.

Even certain cartoon/anime shows like Xiaolin Showdown, Totally Spies!, Naruto, Dragon Ball, Bleach, Fairy Tail, Ranma 1/2, Pokemon, Kim Possible and Teenage Mutant Ninja Turtles (1987) were originally action shows, but had a right amount of humor.

Speaking of which, some serious cartoons had comedic spin-offs that worked such as Rise of The Teenage Mutant Ninja Turtles, Naruto SD: Rock Lee and His Ninja Pals and Batman: The Brave and The Bold, but this show failed at that since it uses poor toilet/crude humor and generic comedy scenes that were bad or boring.

You know you've failed BIG TIME when parody versions of ThunderCats can do comedy better than the actual comedic spin-off of ThunderCats.

Even the MAD sketch 'ThunderLOLCats' and the Robot Chicken sketches of Thundercats, both which are from (intentional) parody shows, did the comedy better, while this show did not.

And the thing is, the originals had a dark tone (possibly more than Teen Titans (2003), The Powerpuff Girls (1998), and Ben 10 (2005) combined, though to be fair, it had lighthearted moments), so any chance of making an action-comedy of ThunderCats would be a very rancid idea, to begin with.

The tone also is very off-place, being way too light-hearted and comedic.

Not only that, but the designs even try so hard to look 'cute' (the announcement trailer depicted Lion-O making the ':3' face), but as said before, it just fails.

This style usually never works for cartoons with anime-esque looking designs or better yet, action cartoons in general. The generic thin-line animation style (or rather, bean mouth style) is used for the characters, which looks way too simplistic and ends up failing miserably but yet typically altering the body proportions drastically.It doesn't have any charm, heart, or anything that made both the original and the short-lived 2011 revival great at all. It instead feels more like a fan-made YouTube parody that was somehow picked up by Cartoon Network due to its massive popularity (keep in mind that this happened with Annoying Orange). Horrendous grasp of the source material: the real issue is that it's far from being an actual ThunderCats show.

Thundercats anime