Yoshi goes through doors all the time in Mario 64 DS.
He's overcome his fear. Or...
Super Mario 64 DS is Yoshi dreaming that he's the hero to rescue PeachIn this dream, Yoshi is able to open doors and conquer many obstacles that would otherwise prove to be a challenge for even Mario. But even in his dream, Mario is the one to defeat Bowser, and rescue the Princess, his subconscious finally coming to terms with the fact that he would likely never be as great a hero as Mario. As time goes on, he must take on the persona of others (via the caps) to even be able to complete the challenges, and finds himself cast aside in more extreme cases to make way for the other heroes to save the day. This comes to a head when even he cannot enter the final Bowser door; that true greatness will forever be beyond his reach. This realisation has a negative effect on the stricken Yoshi, and when he awakes on top of the castle to see Mario with all of the 120 stars (i.e: the original game), he concedes that Mario is the hero (albeit by hiding his insane jealousy), provides him with 100 lives then casts himself from the roof of the castle to his demise.
I need sleep.