The claim here is that vegans look unhealthy. This is typically substantiated by photos and videos of vegans who look unhealthy
The toupee fallacy is an argument that goes "All toupées look fake; I've never seen one that I couldn't tell was fake." You probably have run into vegans that are in great shape and had no idea; they just didn't advertise it to you. Is it reasonable if I run around with a camera filming the morbidly obese and stating that this is what carnism does to you?
Whoa, Whoa, Whoa! "I have a condition...". Sorry, were we allowed to vaguely appeal to conditions? If it's ok to vaguely point to "medical conditions that stop someone from going vegan" then... well.. what about some vague medical conditions that make someone look unhealthy?
Most of these "unhealthy vegans" are people on bizarre diets like fruititarian or raw-food diets. Which, sure, these diets are vegan, but they are also gluten free. Do you post photos of these same people going "This is what gluten-free food does to you"?
People being starved to death (in some cases doing it to themelves) is also ketogenic and equally "technically compatible" with a keto diet because it's low-carb. Does that mean pictures of starving children or whatever are representative of a keto diet? Of course not. Not any more than its representative of veganism. So this whole argument is ridiculous.
Youtube videos of random youtubers that tried veganism for some unspecified time and then quit that don't control for lighting don't represent significant scientific evidence. Scientific consensus represents good evidence. In some cases, Non-vegans are accidentally used in creating content to attempt to dunk on how unhealthy vegans look, as this hilarious video from Vegan Lifting Logic demonstrates.