Customize Controls for each Character.
Just a suggestion, I changed my controller layout to auto attack on RT, dodge/parry on LB, and ult on X fairly early on, took a little while to get used to but now feels really comfortable and I find no problems or difficulty in playing Mekko.
I do perfectly understand how hard it can be playing some characters with the default button layout, and can see some difficulty in Mekko with it also. Maybe try my button layout, is fine for me on everyone but Cass, I find her awkward to play, unless I would change layout or use an elite controller and bind jump to one of the extra buttons.
Just for clarity, I changed my layout in the first place so I could have better camera control while attacking