Does Shaco ult dodge Yi's Alpha strike (if timed right)?

It probably could because when Shaco ults he literally stops existing for a small period of time. Of course the correct timing to dodge is so astronomically small that you're unlikely to see it happen except in diamond + level of play and even then you would still be hard pressed to see it occur.

Seeing that Alpha Strike's damage window is at the very end of his Alpha Strike, depending on the method of cancellation, it's very possible to negate it entirely (at least onto oneself).

Shaco's ultimate dodges anything that isnt instant or already applied.