Fun fact: Yoshi's shield is the only one that cannot ever be shield poked.
Yoshi doesn't really have the right tools to handle Diddy properly, but when your opponent is smart options open up for Yoshi. First, if you ever see a Yoshi in his shield, literally wait for him to do something and then punish it. Yoshi has like no OoS options. If he grabs spot dodge, if he rolls banana him, if he spot dodges punish it. It's so free.
That aside Yoshi can chain grab you so watch out for pivot grabs. Yoshi messes us up off stage and when we are above him so just play careful. He has a lot of jumping techniques (like Draconic Reverse) which can catch items, but its really hard to do and not too practical. He is going to ledge camp you hard, but he has to hit the stage eventually and thats when you mess him up. His main kill move is gone be either up smash or uair, but we've already agreed to stay from above him so just don;t throw at crazy air dodges and up smash is the only thing you will have to watch out for. Yoshi's second jump has euper armor, so keep that in mind if you want to trip him. Lastly, Yoshi's should NEVER do this, but if they screw up and recover low, footstool their second jump for a free stock.
vs.
is definitely a lot tougher for Yoshi and the only way he can win is by simply out smarting you. Play smart and stay safe for the win.
Summary:
1) Avoid chain grabs so that you aren't put off stage.
2) Punish Yoshi anytime he decides to sit in his shield.
3) Watch out for crazy double jump techniques (like DR item throw)
4) Gimp him if he messes up
5) Wait out ledge camping and when he comes back on stage, destroy him.
6) Avoid Up smash and don't get uair because of a silly baited air dodge.
7) Play smarter not harder ^_^