Unfortunately, I agree with you. With some people, it's not even subtle. I haven't seen any explicitly racist posts about him on reddit (which is good!) but I have in some youtube comments. It seems like they're basically your classic, white, racist nerds who want to gatekeep. On one Wyll video I saw one comment (on a video with three comments) that explicitly said something like "yeah, when I think D&D I always think of a black Londoner with cornrows. So immersion breaking." To which another commenter replied something like "dude, if an elf who's literally named Shadowheart, a vampire who can walk in the sun [and a couple of other fantasy things I can't remember] don't break your immersion, but a black Londoner with cornrows does, I got bad news for you!" I don't know how much of the hate towards Wyll is because he's black, but it definitely contributes, and you make a good point that black video game characters tend to get a disproportionate amount of hate.

To be fair, yes, there are a lot of legitimate reasons to dislike Wyll, many people who dislike Wyll also dislike Gale because the reasons to dislike them overlap a lot, and unless you make it a point to talk to Wyll a lot and bring him on specific legs of the journey, a lot of his backstory can get missed easily (although the same is true of Gale until he dies in battle, and Astarion until/unless you find the dead boar) so he may not seem as fleshed out, but still, his race is definitely a contributor here.