Well Cathay has abig wall to protect it and their gods/leader and the children of the gods live with them and are quite powerful, so maybe thats why there is no doom and gloom there. Or maybe the people are lied to so tey think everything is fine.

It’s a bit like the Tau who are also more positive and less grimdark then all the other 40K races and it works.

About the design as onlyonepinman already said the skull motiv is strong in the empire and Cathay was designed with the help from Games Workshop, so maybe they wanted it that way for reasons

Given that the current art work being shown off for old world in the announced Imperial Zoo  RPG book looks like light and fluffy Mantica color and theme wise and that I saw the Total War commercial for Cathay, Chaos, and Kislev not mention this is also the setting with Egyptian mummies and Aztec lizardmen, they will likely fit in just fine (all the older factions seem to have got a less grim drip treatment in the video game, and it was looking like drip by 8th in a bad way so that lighter touch up needed to happen), and yeah they already are established, they showed up  in mordheim as mercs if I recall right and a guy from Cathy is a veiwpoint chronicler  for one of the Lizardman army list books, they are established in world the same way Kislev and Araby are (a few minis here and there with snippets of backstory).

To be honest from a fantasy horror perspective they are a walled feudal Chinese style human civilization that is run by literal dragons that is fighting against all their neighbors (who either want to enslave them, eat them, eat their souls, or some combination thereof). I would say that’s grim-dark enough for all practical purposes and possibly even mildly Lovecraftian given the inhuman ruler angle, plus I get the feeling we will see plenty of internal political backstabbing with Cathy since that is ongoing theme in Chinese and general Asian fantasy (not to mention Warhammer).  Basically I would say they are likely going to be a deskulled counterpart to the empire in oldworld that is probably going to be slightly more draconian,  I will admit it does feel like they stole quite a bit from the fan army books and the Great Wall movie to flesh it out, but that is how GW often rolls.

As to skulls, skulls are not gothic,  gothic is actually a Western European architectural style developed in the middle age that features buttresses and most noted in Christian cathedrals and churches  built at that time. Then the word “gothic” got hijacked by a bunch of Anglo sphere pulp horror writers in the late 18th and early 19th century to describe the genera of their ghost and  sparkly vampire stories which were also very western European in theme and atmosphere and often took place in or near old western style churches. So of course east Asian style  Cathy doesn’t feel gothic, the Aztec Lizardmen and  Egyptian mummies are not gothic feeling either along with most of the other factions such as Kisvel and the dark elves, skulls included.

Frankly the original game wasn’t even gothic to begin with, it was a tongue and cheek knock off of Tolkien’s Middle Earth with a Renaissance human faction replacing dark age Gondor and the only factions in WFB that are “gothic” are the Empire, Brettonia, Vampire counts and arguably (as in “I for one would argue heavily against it”) maybe Chaos due solely to the use of “gothic” style plate armor.

Also  Skulls and skull motifs  are kind of tacky and heavily over used on fantasy and SciFi wargaming minis and I am not the only one who complains about it lol, so I say good for GW for not adding them this time. The 80’s are over and the only ones who should be using skulls at this point are pirates, undead, and maybe “heavy on the human sacrifice theme” factions. (Sorry for going on about it but a bit of a button issue for me).

P.S. You say Brettonia is starving its peasants, my old archer  kit says they are over eating compared to their liege lords, we sure the source books aren’t hiding the truth from us, the truth that the peasants actually control Brettonia and use the nobles as expendable heavy cavalry while they shoot arrows from a safe distance?  This last bit is a joke so take as such but it does seem that way (lol).