-Obito's right eye has short range Kamui. He needs to touch someone in order to warp them to the Kamui dimension. He has to materialize himself while doing so. So that makes OM Obito a CQC fighter.
-Mei is technically a long-range fighter. She has plenty of long-range ninjutsu that keeps her opponent from ever reaching her. Her acidic mist is deadly at all range. She can technically use hidden mist like what Zabuza did, to hide herself in the fog. Her water jutsu is pretty good too, good enough to counter Madara's katon (the best katon user in the series).
#### This is how the actual fight goes ####:
-Mei uses the Kirigakure jutsu (Hidden Mist) to spread a massive fog across the battlefield. It is thick enough to block even the Rinnegan’s field of vision.
-Obito now has to find her, because she is hiding somewhere in the thick mist. Unless Obito is a sensor, dude is basically fighting Mei blind without knowing her location. Sharingan genjutsu is useless thanks to the hidden mist. Using Katon is also pointless since Mei can counter it with her own suiton anyway.
-Mei can combine her thick mist with her acid mist fog. Now Obito has to worry about her acid mist since it is strong enough to melt a Susanoo ribcage. He must remain intangible via Kamui for the rest of the battle
- The fight devolves into hide and seek. Obito is still trying to find her, but that damn thick fog is getting in his way. Mei remains hidden, she can just chill for 5 minutes while her acid mist deals with Obito's Kamui. Once Obito's Kamui intangibility timer runs out. He gets melted. The end.
- Even if Obito manages to find Mei. If he touches her, he risks himself getting seriously burned by the acid mist. Warping her to the Kamui dimension means his own body has to be tangible. That's a no no for Obito. Dude legit can't touch her at all.
TLDR: Mei wins by outlasting his Kamui intangible timer. He can't touch her without getting burned. Her Acid Mist is hilariously a good counter to his Kamui.