Lemon fics in general thrive on pairing dynamics that have a lot of unresolved tension in the source material, and Mei is a perfect storm for that. She's a mature, confident, and openly flirtatious kage in 'Naruto' surrounded by mostly younger, more stoic male characters. The narrative gives her clear chemistry with characters like Kakashi or even a stoic figure like Gaara, but the series never explores it, leaving a massive void. That void is fanfiction fuel. Readers aren't just looking for smut; they're looking for a specific flavor of power dynamics—an older, experienced woman taking the lead, often with a partner who's all about control elsewhere but surrenders it here. The appeal is in writing her OOC (out of character) while still feeling plausible; she's already shown to be bold, so taking it further feels like a natural extension.
Also, I think her visual design plays a huge role. The lavender hair, the distinctive eyes, the whole 'beautiful but deadly' aesthetic lends itself to a certain style of prose that writers love to indulge in. Descriptions become a playground. Plus, her position as Mizukage adds a layer of political intrigue or forbidden romance that you don't get with a regular kunoichi. It's not just a hook-up; it's a diplomatic incident waiting to happen, which raises the stakes and the drama, making the payoff more intense.
Honestly, sometimes I wonder if part of it is also a counterbalance to the flood of hyper-innocent female character fics. Mei offers a character who is canonically sexual without being a victim or a joke, and that's a refreshing sandbox to play in for both writers and readers tired of the same old tropes.