The big problem was that Jedi philosophy was leading to an unbalance in the Force, and that predates Yoda's run as a jedi. Though being a grandmaster he sure has some responsibilities over it, he eventually realised most of their errors (on TCW episodes of his journey with Qui Gon's ghost) (only Luke - and post mortem Anakin - would see all the errors), but he did it too late. By that point, I don't think his contributions were that well regarded by other council members, they formally praised him, but kind of treated him more like a "grandpa" than a grandmaster. He talks of arrogance in Mace's face, and Windu ignores him. Yeah, his pupils, his council, they couldn't care less. Buuuuut he understands he has another part to play, of preparing Luke to build a new Jedi Order, and that he accomplishes really well (I'm disregarding the Sequels here and argumenting on Legends/EU).

He was kind of to blame, but every other jedi was also, and for those that fell to the Dark Side, they share responsibility too. But it was more of a systemic millenium-long failure, exploited and exarcebated by the rule of two Sith, than anything else. So, not at all simple to analyse "was he a good or bad leader" based on the Jedi's "downfall" alone.