She feels threatened. Right after Mari talked to Shirase she made this dumb starstruck face and started to talk non-stop about Shirase to Megu. The cowardly Mari that can't even bring herself to skip school for just one day, the helpless Mari that never seems to clean her room. That same Mari is now talking to her so passionately about a weird girl that everyone laughs at. Megu felt her existence in Mari's life outshined by Shirase, and she doesn't like it.
Megu probably started spreading rumors about Shirase's 1 million yen without thinking too. She's pissed and she doesn't know why, so she took it out on Shirase, the weird girl that got Mari to talk non-stop about nonsense like going to Antarctica.