Sylvanas didn't plan the Wrathgate, however:
a.) She did order the Forsaken Blight to be created. We saw during that and Hillsbrad's quest chain in Vanilla that the Blight, at the least could kill Humans and Undead at the time of those tests, and didn't apparently keep up with the development of the Blight to see it somehow being developed to kill Orcs and Trolls and Tauren, ect.
b.) She put Putress in charge and seemingly didn't keep an eye on him or what was going on, which is uncharacteristic of such a controlling character
c.) She allied herself with a Dreadlord, perhaps the sneakiest and most manipulative Demonic race that exists and didn't keep him on a short leash after his short stint of probation in Frozen Throne.
Although all of this ultimately falls on Thrall's shoulders as the unilateral dictator (He may be nice but the Warchief is a dictator position) of the Horde; letting Sylvanas do whatever she wanted until it blew up in her face.
So, no, she's definitely not responsible for the Wrathgate's planning and execution. But her incompetence, willful ignorance, or disinterest in the activities of her own forces ended up biting her right in the ass and made her even more distrusted than she already was.