Theres no direct quotes on that...The Pixies arent even the biggest influence on Nirvana overall, they really share more similarities with The Melvins and Black Flag than they do with The Pixies.
I dont really hear any real similarities between the two myself, the vocals and the guitar are not at all similar...And the songs are not quiet as agressive as Nirvanas.
I dont consider Nirvana a ripoff of The Pixies, they barely sound alike at all, and if Kurt Cobain didnt go on about how he loved The Pixies i doubt anyone would be making such a comparison, to me its just a bunch of elitist douchebags making a excuse to bring Nirvana down.
The only real similarities is the soft/loud dynamic, and Nirvana took this format form from The Pixies and took it to a whole new level, by making the music much louder and harder...And they made this style much more popular.
Either way, its NOT like The Pixies were the first band to do the soft/loud thing, Led Zeppelin have done it with several of their songs...King Crimson and Jethro Tull have done it too.
Back on topic, The Beatles had the most impact of any rock OR pop band.