Wouldn't it technically be "Queen Koopa" since normal Bowser is a king?
The original commentary says Koopa-Hime (ピーチ姫) with Hime translating to Princess. If you're asking for why she's often called that in Japanese, then it's probably because the original power up that started the Bowsette trend, the Super Crown, originally transformed Toadette into a Princess Peach lookalike.
Wouldn't it technically be "Queen Koopa" since normal Bowser is a king?
It's because it's an item that transforms the character into being more like Princess Peach. So, Toadette -> Peachette. In Japanese, Toadette is "Kinopiko", which becomes "Kinopiichi".
In Japan, Bowser is "Daimaou Koopa" (Great Demon King Koopa), so they just change his title to match Princess Peach's.
