Mirri Maaz Duur herself did say something to the effect that Drogo was a brute, that he'd killed many people that she loved, and that now he (and his son) would hurt no one else. However, I would not say that it is crystal clear that she did kill him.
She did claim that the khal did not enjoy her first poultice, which she claimed was a "cleansing fire". This rings true enough to me. I believe she applied the "soothing" poultice, but whether this is what killed him, I do not know. It might be that she attempted real healing first, but when Drogo objected, she simply thought "Fine, go ahead and kill yourself, you murdering scum."
It is clear, however, that she betrayed Daenerys when she said she could bring him back. As evidenced by her own words to Daenerys afterwards: Now you see what life is worth. Implying that once you lose everything that makes life worth living, life itself is meaningless.
She turned Drogo into a zombie, indirectly killed Daenerys' unborn child, and gloated about it afterwards. It is clear that Mirri harboured a profound resentment against the Dothraki, and that she did what she could to exact a gruesome vengeance.
But most importantly, she was the catalyst that woke the dragon in Daenerys and inspired her to perform her ritual that hatched her dragons. If not for her, that might never have happened.