I can't tell if Mari is wearing one of those shirts that doubles as a dress, or if she's simply not wearing any pants/shorts/skirt. Not that anybody's complaining!
Right?! I was absently wondering about this detail as well. I thought, "Wait, was she wearing shorts before, or is that like a sweater/skirt?"...or more importantly "Was she just not wearing anything down there to begin with?" and then "hmm...drafty", finally I settled on "Do women often not wear bottoms on buses in Japan? And if so...why don't I live in Japan, again?". Ah, the inner ramblings of a yuri fan-girl.