Under rated by a margin. She is so not Jounin level, i would say low-mid kage for sure. Yes, Kakashi and Gai can beat her but we have seen loads from them and they seem quite OP, (Kamui and 8 Gates).
Lava Style able to melt Susanoo and being commended by Madara Uchiha to be of harm if in contact. Boil Release able to also melt Susanoo amd can be used in open area but not as effective in a smaller space. The only reason she cancelled it at the Summit, was not to harm the other Kage. And Karin did not dodge her lava! First time, she aimed for the hole to stop them from leaving. Second time, she aimed for Sasuke. Not aiming for Karin so Mei did not miss
Also having Fire, Water and Earth techniques. Having only seen her Water Techniwues which were able to counter Madara Uchiha's Fire Style, with Uchiha's having the best Fire Style, and Madara being extremely powerful, shows how powerful her ninjutsu is.
Mostly under rated as we have not seen much from her compared to the other Kage. I am not sure if she is the weakest, as in my opinion she has a chance against Gaara who is one dimensional, but people will argue against that.