Was it ever relevant? Popular, sure. But relevant?
Relevancy is a relative quality. We don't say that something simply is or is not "relevant." It needs to be relevant in relation to something else. Usually some standard or topic.
Recency and popular culture are common guideposts to which we peg relevancy. If we choose to peg relevancy to 'current pop culture,' then we could say Angry Birds is less relevant today than it used to be in its prime. And we'd probably say that Angry Birds was quite relevant in its prime, being the premiere game in the breakthrough period of mainstream mobile gaming.
If we choose to peg relevancy to an industry standard -- say, 'current best practices in the gaming business' -- then we reach a similar conclusion. Angry Birds was hyperrelevant in its heyday, but less so today.