I don’t think that it was Michale Gambon’s fault that his version of Dumbledore sucked, but it did. I think it was the screenwriters fault for making Dumbledore so crazy and wild in movies 3-8. I hate it when blame is put on the actor and the actor only for the character they play (especially when it is adapted from a book) when the character is adapted horribly.
That being said, I do think Richard Harris did a better job portraying Dumbledore than Michael Gambon in general. Richard Harris is exactly how I pictured Dumbledore looking.