Well, it really depends. Against Charizard X and Y, does Yoshi get his Super Dragon power to level it out? If so, he might stand a chance. Although, as great as he is normally, he probably couldn't rival the speed and attack power of normal Charizard. Yoshi might have the egg powers and the flutter jump, but eggs are limited and Charizard can flat-out fly. Against Charizard X, any physical attack (Outrage, Flare Blitz, Mega Punch, etc.) would probably OHKO (like in competitive Pokémon.) However, Yoshi's range might allow him to win against Zard X's physical compliment. Zard Y would be more of a challenge, with Drought Fire Blast, which is probably the most recklessly powerful attack you can do (other than Overheat or Blast Burn, that is.) However, Super Dragon Yoshi has more evasion and range, having flight and fireballs. He could take advantage of Zard X's close combat fighting style, and use a fire ball (boosted by Zard Y's Drought.) Basically, Charizard has a better standard form, while Yoshi has a better powered-up form.
