Honestly it seems that Mei in terms of potential is better than A. In execution of potential however, she falls behind A. So she has the capabilities of being Mid-High Kage but it currently stuck at low kage level.
Mei is low kage she has impressive Releases but they can be easily dodged like how karin yes KARIN dodged them easily by jumping... They are offensive but there easily avoided..
Or high joinin
no such thing as weakest kage. they all can defeat each other given right conditions. and btw how do you know that lava stream was going as fast as possible? it definitely hit and melted madaras susano. mei has one of the most impressive arsenals so far of anyone in the series. best suiton aside from kisame, acid mist, blinding mist and lava.