My opinion might be kind of mean/controversial, but I think it's basically impossible for most music fans (of both humans and vocal synths) to NOT care about the appearance of the singer. Like, I think it's natural that if you become a fan of a singer's voice that you become curious what they look like because you want to learn more about them.
Imagine you are a fan of a human singer and finally Google what they look like and to your horror, they look just plain weird! (In your humble opinion.) Like... green 7 ft tall hair, one single diamond-encrusted tooth, a big tattoo on their forehead that says "MuNNie$" on it, they wear backwards clothes for some reason... Don't you think having your mental image of them "shattered" affect your feelings as soon as you hear a song by them, even if they were super awesome until you Googled them? They probably seem a lot less cool now.
They say that people become fans of bands/musicians because their image (clothes, attitude) and message reverberate with you (for example, a band that is considered slightly "hokey" by being extremely patriotic in the middle of every single concert by asking veterans to stand up followed by a "God Bless America" cover, but you just eat that stuff up and are part of their "fan army" that has a cool name).
So, back to vocal synths. I feel like it's hard to care about a vocal when you are presented with a sterile software cover that has NOTHING to base the singer's attitude/image off (like VY1, VY2, Leon, Lola). How are you supposed to know if the vocal will jive with the type of music you want to produce? Just by reading the specs and hoping it'll work out?
Another difficult situation is being presented with very questionable box art that doesn't scream "super cool" (ex: Gachapoid, Sweet Ann). Personally, I like to draw and can do design tweaks for Vocaloids, so I guess even if the official art is "oogly" I could fix it to my liking. But I think that EVERYONE likes a cool music video to go along with the singing (fans and producers). If you drew something really ugly for the music video, it would be like the 7 ft tall green haired diamond tooth singer I mentioned earlier, no one would want to sit through that type of video, now would they?
I guess it comes down to art/music for the sake of art OR "I want to make money". If you don't care about your image or how people perceive it, fine, make whatever or use whatever questionable box art to promote it and just release it into the wild, what happens happens. Just don't expect people to relate to it or come flocking to it.
But if you want to promote your work with the goal of making money, you have to be flashy and try to catch the attention of people who will relate to the character and want to learn more. That's just how it works in show business.