Innocently, Vonnegut took much of his information on Dresden from David Irving, an historian who has since been widely discredited as a Holocaust-denier and Nazi apologist. Many of Irving's claims on Dresden have been discredited (for instance he overestimated casualties by tens of thousands, and downplayed the tactical significance of the factories that were bombed).
Does this innocent error discredit Vonnegut's book? No way. Mass slaughter is a reality, and the emotional response to it is real. The political games played around that slaughter by those with axes to grind make no difference to the power of the book. A discussion of Irving, Dresden and war crimes on Crooked Timber.