Yoshi's Dash is very good in general, it's fast, low profile, and he gets a lot of options out of it. Dash attack vs Dash Grab is a classic mixup, and both Yoshi's Dash attack and Dash Grab are good so it's threatening enough to force the opponent to guess. Run up UpSmash is dangerous and should only be used to punish unsafe landings IMO. Run up Shield is also super good in this game. Obviously, shield grab or spot dodge is an option if the opponent does an attack, but even if not, you can just instantly shield drop into Ftilt, Dtilt, FSmash, or DSmash. Shield Drop DownB is a kill move if they hit your shield with something laggy. He has good aerials to use out of a dash too, depending on what the opponent does. Dair eats spot dodge and can be safe on shield against some characters. Nair is solid if the opponent tries to challenge with their own aerial. RAR Bair is a good aerial chasedown if they try to escape the situation. Jump back Egg is available as a fakeout or if you think they're going to challenge with something scary.
Overall, the key is to mix it up until the opponent exposes a tendency, then try to go to the option that beats what you expect them to do. Yoshi has tons of options so you should try not to become predictable.