Important yes, 1/3 of the game no. There are far fewer special teams plays than offense or defense, so the yardage totals are a lot less. For example, the Rams this year have 4,779 yards total offense (rushing and passing), but only 1,170 in kickoff and punt return yards. I do think that a great kicker, punter or returner can be a huge difference-maker, but the coverage guys are way less important than the regulars on offense and defense.