Theramore was a valid target. Garrosh showed real military genius in doing what he did. Launching a bomb isn't necessarily cowardly.
BUT...a mana bomb is no ordinary bomb, and the reason they labelled it as a WMD isn't because of a blast radius, but because it literally obliterates the surrounding area from existence. There's no ruin, no way to grieve for the lost, no way to even give them a proper funeral. As well, when I say obliterates the surrounding area from existence, I mean if there were 100 copies of Azeroth in alternate realities/dimensions/whatever, the other 99 theramores would have been obliterated at that moment in time as well.
While I agree what Garrosh did showed he knew his military strategies, drawing the alliance in while taking his own men out, a mana bomb that does what it did? Yeah, it's a WMD, and cowardly (and I'm horde myself).