I don't think ZSS would beat Yoshi from a gimping standpoint, Yoshi's also very skilled at coming in high or covering his recovery with eggs, and then there's armor too. But in neutral I can see her beating us with really well-spaced nairs, just enough to keep safe and out of Yoshi's jabs and nair oos.
And if we get in shield a lot, Yoshis do like the shield, ZSS can get a lot of percent off a throw, and Yoshis going autopilot you can up B out of shield, but we have a command grab for those who love the shield too much....and so on and so forth. It seems to go back and forth pretty evenly in my mind, at least, but neither side can really relax all that much. So, yeah, my vote is for relatively even, but volatile.