She isn’t much of a mixup character (though she does have some damaging mixup with meter). The problem is her normals are too good and it’s not really an issue of “how do I block this?” as it is “when can I do something other than block?”. Now R-actions can be a solution to a lot of problems but the problem here is that Mitsuru has probably the most damaging combos in the game (I suppose Elizabeth’s are more damaging). You can die in 2 Mitsuru combos most of the time, so it’s quite risky to make reversal guesses to get out of her pressure.
Her r-action is very good and it doesn’t make her go high enough in the air so it’s harder to punish than it should be. You have to punish it while she’s in the air if she has meter, or she can supercancel it when she lands…if the r-action is airblocked, there’s many cases you won’t be able to punish it in the air.
Her risk/reward is greatly skewed in her favor. Worst of all, she is very easy to use and in most cases you don’t have to actually think about what you’re doing because you can cause many problems for the opponent by just doing moves (and in most cases you can cover a number of situations with single button presses, without thinking about them). The opponent has to go way out of their way to counter simple brainless things that she might do.