"Instantly important" at least in the sense that they broke genres and movements so hard that they were forever associated with them.

"She Love You" changed the world.

Nirvana is not even close.

Kurt Cobain said he was heavily influenced by The Pixies. I personally heard of The Pixies and Throwing Muses around 1988, and I was convinced a musical revolution was underway at the time. Both bands were joining the likes of The Smithereens, The Godfathers, The Pursuit of Happiness, the Sidewinders (who later changed their name to The Sand Rubies), They Might Be Giants, King Missile, and a few others I don't think I could put in the same class.