Man, this is my second read of this manga + I have to say, rereading this part while knowing everything just made me even more in awe of how incredibly Mami is written...
Both characters and readers fall prey to the tropey bullshit we're all used to ("boys and girls can't be just friends" in real life, "annoying rival" in fiction) and it's the characters own insecurities and biases that make Mami the villain, not her actions. From the moment she was introduced, she has done literally NOTHING suggestive whatsoever. It's insane how we are conditioned this way