It's not even the fact that he's fat per se, it's that he has no physical capacity at all. There are fat martial artists, wrestlers, strongmen... that can outperform leaner or more muscular looking guys.
It has more to do with Ethan being a pathetic slob that probably gets winded just from standing up, has erectile dysfunction before reaching 35, doing stupid shit like trying to make it look like Joe Rogan is as fat as he is (ignoring the fact that Rogan is 20 years older than him), and his mental state being damaged by his fat body, poor diet and lack of exercise.
Add to that how judgmental he is of everyone and how he loves giving advice on how to stay healthy, there's no wonder people are calling him out on it. Making fun of him to push him to change is not the right approach but it's miles better than being like "it's fatphobic to criticise him, leave him be!", at least the people mocking him won't be complicit of his death of heart failure before reaching 50.