You usually have enough time to react to his Down-B when Yoshi performs it, so you could dashdance (or just run if you are stationary) away from him and still get back to Yoshi with plenty of time left. The stars stay out until frame 16 after Yoshi hits the ground and the landing lag is 40 frames, so that means you have a good 24 frames (a fourth of a second) to get to Yoshi and punish before Yoshi can do anything.
EDIT: Also, if you are right next to Yoshi, don't spotdodge. Fox's spotdodge is only 14 frames long, so the stars stay out too long for that to be viable. It could work if you timed it really well near the edge of the stars, but it's more risky for no reward.