In many areas (class design especially) I think it was. Overall I always felt like WoW was improving up until the end of MoP, despite the odd misstep along the way. They might've made the odd bad choice here and there, but for the most part the game was being iterated upon and expanded in interesting and compelling new ways that showed a lot of promise for the future.
The WoD-Legion period has had some good quality of life improvements too, and Legion in particular has many systems that are definite steps forward, but the *overall* direction of the game feels to me like it's been meandering in experimental and somewhat gimmicky directions rather than fundamentally improving on the game at its core. Garrisons and Artifacts, for example, feel like completely temporary gimmicks that serve to stimulate engagement with their individual expansions without adding much to the game long-term. The handling of reward structures in WoD was shockingly poorly done, and Legion hasn't yet managed to solve all of those problems either. You also have things like the treatment flying's been given, which continues to prove far more contentious than the alternative that existed prior to WoD.
I feel as though WoW is an experimental era right now, with the devs trying out a lot of significant and often gimmicky changes to see if they can strike gold with something. It's a little concerning to me, since a twelve year old game should probably be beyond such inconsistent swings in design at this point. The spec revamps we saw in Legion are probably the biggest example of this, seeming to have been guided by a mindset of "let's try this out" as opposed to "let's make this better".