I searched in Rambam hilchot Korban Pessach and if I don't miss something, no Kappara (atonement) is mentioned.
Thanks to the answer of mevaqesh we can see here the Tif'eret Israel who is astonished by the Tosfot Yom Tov comparison between Pessach and taanit about Kappara. The astonishing of Tif'eret Israel is perhaps also due to the lack of sources.
But after an accurate lecture of Tosfot Yom Tov, he also not said that the Pessach is Kappara . He said a comparison between the sets: the set of korbanot and the set of taaniot (not a comparison between the two items Pessach and taanit). The major feature of the set of korbanot is Kappara and the set of taaniot also. So Pessach as an item of the set of korbanot which is mainly characterized by capara. So the name capara is linked to pessach by his specie.
But the Levush Hachur OC 468, 1 also found by @mevaqesh doesn't support this teruts. Anyway here It is probably right to answer that to make himself exempt from caret because bitul mitsvat asse of Korban Pessach is the called Kappara but not at the sense of taanit.
So as suggested by the lecture of Rambam (and Gemara and Tora ) Pessach is not a Korban Kappara and even the authors cited as thinking that it's Kappara don't really said that.