As for going against Yoshi it depends on how that person plays. An agressive Yoshi that attacks constantly can easily be defeated if you block their attack and then counterattack. You can even use projectiles to counter Yoshis that don't use eggs often. As for beating defensive Yoshis like myself, you should try to make it difficult for Yoshi to use his eggs. One way to do that is use a character that can reflect attacks. That prevents Yoshi from using eggs so freely. Using characters with faster projectiles can help too. One of Yoshis hardest matchups is with ROB because his projectiles are much faster than Yoshis and he can reflect too. Another way to beat Yoshi is to shield grab him. I admit that I often use aerial attacks to approach often. His air attacks are much faster than his ground attacks. Another way to make the battle is to play fairly close to Yoshi to apply pressure and make it hard for him to use eggs or aerial attacks.
I always have a lot of trouble against DDD. He is too fat and not easy to approach. lol In all honesty though, I don't really have much tips I can give you. Ever since my match with Prime about 3 weeks ago, I have not been playing Smash too much lately. I'm either trying to achieve a high battle rating in MK8 or I am just working. I recently got a full time night job so I haven't had nearly as much time to play. On the contrary, I believe that I am the one that should be asking you for tips. 
