oh jeez.
I do not think it's inherently bad to dislike depictions of Vocaloids. Hell, I've had negative reactions to songs on how they portray the character in said song. I don't think you are incorrect for having this specific point of view, especially if you don't like deviations of characters in other aspects (Like their age or their height).
However, I do not think arguing the point of disliking racial and body type headcanons with lies about the people who make art for themselves and how doing that is wrong is something that should never be encouraged. And I do not think it fosters any positivity among this community; a community founded on creativity and expression.
On a related note, there's aren't a lot of Vocal Synth characters (especially commercially) who aren't pale or slim. Even English voicebanks made in countries with a high black population don't have a lot of black characters, and the only commercial voicebank with a notable design who's plus sized I can name is NYL.
A large reason why people "race-wash" popular characters is because they love the character and like to see traits they do not see in media that often in a character they love. These traits can be based on a person they love, or to see themselves in that character. They can even be for fun! I think if someone wants to take Miku, a Vocal Synth who's company has openly encouraged others to portray her however they want, and make her black for an artwork no one is paying them to make... They're not doing anything wrong and not harming others, especially when they're not being an ass about it (the "I fixed this character" argument and only changing traits to make jokes at their expense). People can keep Miku pale, skinny and Japanese if they want as well, and they're not doing anything wrong either.
A lot of the fans who do put headcanons on fictional characters in any fandom do make fan characters! However, a lot of artists that make fan characters get a lot less attention and discussion around their art of fan characters then headcanon's on preexisting characters. And making an UTAU requires a lot more work then just making and drawing a design. It isn't easy to make a whole voicebank, and most won't bother when they just want to see a fat person in the media they love.
I haven't even mentioned the extremely racist and fatphobic arguments at play. Black characters have had a much longer history of being changed to have lighter skin or even written out entirely in officially licensed adaptations, and have many more offensive portrayals that reflect the real worlds aggressive hate for black people to the point of constant social barriers that decide if they can get the essentials to live or not.
Fat people also don't have a lot of good portrayals in media, and the stereotyping seen everywhere in movies and television justify fat people being unable to access proper medical care without going on diets. Which don't solve any medical problems, just create new ones from malnutrition. Both are things I encourage looking into, especially in Japan. Colorisim and fatphobia aren't just an American issue, and Japan is equally capable of cultivating a hostile environment, even if it takes a different form.
Pretty much all Vocaloid material is fan-made. No Vocaloid fanart has the power to make CFM go "Actually, we should make Miku fat!" on an official scale on a whim. Speaking generally, if someone does not like artwork of Vocaloid's being black or fat, they can just look at thousands of others where the characters look more accurate to how they look in canon. While a couple of bad eggs will claim that their personal design is how a character should be, it should not take away how drawing and writing should be fun, and how people's differences are what makes us human.
