Is Cassiopea really a hyper carry mage?

I just played a game where Yas destroyed us, so Im not going to lie, I feel bitter. But in the start (before he was fed), there was an instants where I waited in the bush for yas to return from ganking bot. I stunned him, grounded him and proceeded to E. I was sure I would win giving that I landed everything along with the element of surprise.

It wasn't enough to kill him before the stun duration was over tho. So I knew he would be able to windwall. I proceeded to get with melee range to ensure my e would not get blocked feeling confident he is still grounded and I should be able to finish him off.

This yas, proceeds to charge his Q, knock me up, ult me, kill me, all before the remain poison damage from my Q gets blocked from him shield that had been refreshed from his ult.

Now I don't know about you, but landing skill shots against Yas is hard. It feel unfair that this guy can take so much damage before dying.

Or is it that Cass doesn't do enough damage?