There aren't many characters in the series who have never told a lie... but Mirri, in her brief career, was one of them. Seriously, I don't think she ever said anything that wasn't true; at worst, she just didn't offer information that someone neglected to ask about. Why would she lie to Dany now? Her revenge is already complete. (Besides, it's not like Mirri's statement is the only reason to think this is true; what happened to Dany's child suggests pretty strongly that she was exposed to the magic equivalent of radioactive waste.)
Mirri's revenge is perfect because Drogo and Dany could've avoided it at any point, if only they weren't the way they were. The medicine she prescribed for Drogo's wound is (as far as we know) the real deal, and she stresses how important it is to follow her instructions; but Drogo, predictably, is arrogant enough to ignore the instructions, and Dany, predictably, just goes along with Drogo and hopes it'd be all right. So he ends up with a fatal infection. Dany asks Mirri to save his life, and Mirri says it's possible but it'll require sacrificing another life. Dany doesn't ask her for the crucial details-- which is also predictable, because Dany has bought into the Dothraki's confidence that the khal will always win somehow and that his son is destined to rule the world. And Dany can't imagine that someone whose life she saved wouldn't have her best interests at heart, because she can't imagine that anyone could think slavery was a fate worse than death; Dany has been thinking like a slaveowner.
Mirri is doing what beaten-down workers have always done: "work to rule"-- that is, follow the rules and do exactly what's asked of you but don't go above and beyond, and don't bother arguing with your boss's stupid decisions even if you know they're going to bite him in the ass.


