According to Oricon news, the ED song of "K-ON!" ranked the 2nd in the weekly general single CD ranking. It has been at the top in the daily single CD ranking for three days.

Here is the weekly single CD sales ranking for April 20th - 26th. non-anime songs are in parentheses.

*1, 98,033 One Piece OP ED "Share the World" / "We Are" / "Asu wa Kurukara" by TVXQ
*2, 66,910 K-ON! ED "Don't say 'lazy' " by Sakura High K-ON Club
*3, 61,921 (Aitai by Yuzu)
*4, 61,608 K-ON! OP "Cagayake! GIRLS" by Sakura High K-ON Club
*5, 35,012 (THE MONSTER ~Someday~ by EXILE)
*6, 27,126 Cross Game ED "Koi Kogarete Mita Yume" by Ayaka
*7, 14,865 "Shuuketsu no Sono e" by Hayashibara Megumi
*8, 13,761 (Niji by Kobukuro)
*9, 12,178 (Yaruki Hanabi by Watari Roka Hashiri-tai)
10, 10,629 Hetalia Axis Powers Character song vol.2 by Yasumoto Hiroki as Ludwig

Below top 10

Sources:
Oricon news
Mainichi
Oricon YouTaiju Service

Ref. The first week sales of the recent anime songs. (the ranks in the first week were indicated.)
2, 65.907 Lucky ☆ Star OP "Motteke! Sailor fuku"
3, 56,455 Macross Frontier OP "Lion"
4, 45,324 Mahou Sensei Negima! OP "Happy☆Material (5)"
3, 42,714 Macross Frontier "Iteza Gogo 9ji Don't be late"
5, 34,881 The Melancholy of Haruhi Suzumiya ED "Hare Hare Yukai"
5, 34,501 Macross Frontier "Seikan Hikou"
3. 33,387 Mahou Sensei Negima! OP "Happy☆Material (4)"
3, 31,270 Macross Frontier OP "Triangler"
4, 31,168 Mahou Sensei Negima! ED "Oshiete Hoshiizoo, Shishou"