Okay, I tested it. Time for some math again! (you guys know how much I love grab releases) It turns out it works, but it's not bad at all. Here's why...
Yoshi has 30 frames of immobility after a grab, and Jiggs has 49 after being released. So...
49 frames of immobility - 30 frames of immobility = 19 frames of immobility
Yoshi's jump startup speed is pretty average, and I'm pretty sure that Yoshi's jump takes 7 frames to leave the ground. So if we factor that in...
19 frames of immobility - 7 jump startup frames = 12 frames of immobility
Yoshi cannot use his Uair on the first possible frame, since it won't reach Jiggs. Bowser could throw out his Uair at the first possibe frame, and the trajectory of his jump and the huge hitbox of his Uair would still hit Jiggs. However, Yoshi's Uair does not that the same range or the same amounts of startup time that Bowser's has, so he cannot throw out the Uair at the first possible frame. It simply wouldn't reach Jiggs. He has the delay the Uair slightly, use that time to rise up with his jump, and then throw out his attack. I think that it requires about 4 delayed frames minimum in order reach Jigg's hitbox with the Uair. So...
12 frames of immobility - 4 frames of jumping = 8 frames
Yoshi's Uair comes out in only 5 frames, which is pretty darn quick. So when we do the math...
8 frames of immobility - 5 frames of Uair startup = 3 frames where Jiggs can be hit
Many thanks to firemario149 and Ref for the frame data
From all this ******** math, it looks like Yoshi has an even smaller window of opportunity to use his Uair then Bowser, give or take a frame or so. However, unlike Bowser, Yoshi cannot buffer his Uair out of his jump, since his Uair has to be delayed in order to hit Jiggs. This means that human error is very easily factored into play, since most players do not have frame perfect timing. Most likely, the Yoshi player will not have practiced this technique at all, and they most likely will not have the timing down. Even if they did practice it, the Uair is still more difficult to land on Jiggs then Bowser's Uair is, due to the whole buffering thing and that Bowser has the 10+ frame advantage over Yoshi. Seriously, 3 frames is such a small window of opportunity.
To put it simply, Jiggs shouldn't be scared of this. Remember all the talk we did in the Bowser discussion about avoiding the Uair? Use it here. Going back to what illinialex said a while back, if you airdodge and jump as fast as possible, it'll be very difficult for the Uair to hit you. (Funny how talking about Bowser helps in the Yoshi discussion. XD)
So really. Unlike Bowser, it's not that scary. Have fun guys.