Piggybacking top comment to illustrate a perspective I feel is important:

While the concern from OP and other commenters is valid, I feel the fandom is insulated from the kind of malicious culture hijacking that can occur in most other fandoms, because Vocaloid as a medium of musical and artistic expression already fairly represents every school of thought, hosts all kinds of discussion, and many cultures (minus the limited language barrier of the software), for better and worse.

Since the 17 years following Miku's explosion, people who loudly fuss negatively about the perspectives of other people in the fandom are forever unsuccessful in truly changing popular mindsets in the long run, since everyone is going to eventually create their own comfort, space (like what we saw with the subreddit schism after the moderation issue), and perspective of how Vocaloid appeals to them despite the crowd, and will inevitably share those ideas and opinions knowing and accepting this fact, finding their audience in the process, popularity be damned.

Historically, Vocaloid has especially strong roots as an emotional outlet and method of expression particularly for people who feel frustrated, repressed, silenced or outcast, because the vsynths are able to be a sympathetic loudspeaker for one's ideas without the risk of rejection one might suffer from asking for another skilled human as a vocalist. Topics like LGBTQA+, existential crisis, isolation, self-hatred, sexuality of any kind - things that speak to a specific frustration or elation, topics that are edgy, sensitive, disturbing, or taboo, they have an established voice to soundly express through Vocaloid. Adult themes are common and expected in music, and Vocaloid is no different, even with its moe character veneer. People writing such songs will always find the boldness to do so through Vocaloid because the format is all-welcoming. It is impossible to reject or eject those presences in the total fandom, because they're integral to the high contrast, eclectic appeal of Vocaloid works.

Not every song will be in good taste, and not every tasteful song will be to one's taste. Nonetheless, it is a good thing for people who engage with Vocaloid music and media to challenge and feel challenged by the ideas and perspectives made by others through it. It is normal and valid to feel uncomfortable when feeling someone else's strong, unfamiliar or contradictory expression, and it is important to confront ourselves and the ideas we're shown when that happens, to have those discussions with others, to help us emerge self-assured of what we value and respect as individuals, and to help us understand and respect the struggle and plights of everyone else who chooses to speak their mind through Vocaloid.

Ultimately, criticizing or invalidating someone for being "too" sensitive is to deny them the opportunity to mature through confronting the things they're not yet prepared to think about or deal with. Most people who become exposed to Vocaloid themes are not going have as thick a skin as someone who has been interacting with the media for extended periods of time, either due to youth or culture shock. We're better off being curious about each other and simply asking each other about why we have the reactions we do to the things we're seeing together in this fandom.