wedding dresses are not a new thing in
japanese media
They're pretty standard in the rotation of office outfits, bunny/cat outfits, china dresses, yukatas, swimwear, school uniforms and whatever other common outfits anime go out of their way to put their characters into. Pretty much any gacha game will at some point have wedding dress variants of their characters as well.
Now it does become a bit weird when you're talking about vtubers because of they're real people and the parasocial thing above the "normal" waifu thing. But its also not the first time, infamously Rushia's birthday merch was a wedding ring.