This writing is kind of going to serve as a piggyback/extension to the "all mudwings are fat gives me weird vibes" confession that was submitted here, as this was originally going to be a reply to that confession... and then it got long 😭
Basically, I also find it odd how people can insist on ALL MudWings being fat. I do believe that the average MudWing is bigger (since that's how they appear in the guide), but just like with body types in real life, there are exceptions to the average. Now, if most MudWings are being drawn as skinny, I do think that raises a valid concern of fatphobia; however, portraying a skinnier MudWing every once in a while should not warrant criticism or even an explanation.
Here's the other thing, though: There are other tribes in the guide that also appear to have some fat, such as NightWings. And yet I don't see many general fandom portrayals that reflect this... and I don't think I've ever seen someone criticize those portrayals (for not showing enough fat). Instead, this controversy largely centers around MudWings.
Why is that? Is it because NightWings aren't as associated with food as MudWings are? Is it because they're perceived as intelligent, elegant dragons, and elegance/intelligence and fatness couldn't possibly mix? (That last sentence was sarcastic btw)
Of course, I also understand that modern NightWings suffered from starvation on the island, and a lot of those NightWings likely do appear skinny (but do note it is possible to be malnourished and not loose body weight). These NightWings then relocated to the Rainforest Kingdom, though, and while it can take a while to heal from starvation, many NightWings will probably return to the average body type with time.
I can't really think of a reason to portray so many ancient NightWings as skinny, though... Having fat might even be respectable in NightWing culture, as Listener's fat (yes, she is canonically described as having curves) is noted as sign of being a successful hunter (not as a sign of laziness or incompetence). I'm suddenly wondering if skinnier, starved modern NightWings feel insecure about their bodies...
TL;DR: I feel like many people are only get so weirdly defensive about MudWings being fat because a couple of MudWings were mentioned to like food, and MudWings are also stereotyped as sloppy and dumb. Most of these people conveniently allow the other tribes to be portrayed as skinny, despite some of these tribes actually having more fat. I believe this is because these tribes align less with gross stereotypes about fat (such as liking food and being dumb, as previously mentioned). Essentially, being fat should not be so confined to MudWings.