Also, remember Dash attack likes to cross up shields, so if you play in lag and see it hit, unless he started it early assume Yoshi has crossed you up for when you make your punish attempt. Also, it lasts for a fixed time, so if he hits the ledge he doesn't get as far and the endlag is long enough he can be punished hard.
Dair loses to everything. Especially swords.
Swords actually beat Yoshi bomb if timed perfectly, but I wouldn't try it with anything except Link's Usmash and Uair, which are slightly both risky.
Honestly, the Yoshi's who spam DA and Down air are usually not very good. Focus more on staying safe, watching for patterns and adapting to the lag to play consistently.
Short hop Down air is one of the signs the Yoshi boards uses to spot newbie players. Most of the guys at that play level are just looking to hit with strong moves (roll, smash, sh->dair, dash attack. seem familiar?) because they aren't good enough to use something better. Pick a character you like and focus on playing consistently, not letting the lag get to you and playing your strengths.
Oh yeah, and while rolling beats short hop down air hard, rolling backwards can get you hit by DA. Yoshi is more safe if he guesses a forward roll and retreats the short hop down air to catch you in the endlag of the roll. In short: don't roll.