In "My Hero Academia" (MHA), Mirko, also known as the Rabbit Hero, has not been specifically shown to have been experimented on in the same way some characters have been. Mirko, whose real name is Rumi Usagiyama, is known for her powerful quirks and strong fighting ability, which come from her quirk, "Rabbit," that grants her enhanced physical abilities.
While the series does explore themes of experimentation and how certain characters have been affected by it (like the League of Villains' experiments on Quirk users or the quirks of some Nomus), Mirko's backstory primarily focuses on her determination, strength, and the path she took to become a Pro Hero rather than any experimentation.
If you're looking for specific details or developments regarding Mirko, please feel free to ask!