I had just made my Christmas songs list and from it my favorite J-pop songs are;
B'z: Itsuka no Merry Christmas
BoA: Merikuri
Bonnie Pink: Chain
Crystal Kay: Happy 045 Xmas
J-FRIENDS: Can you feel this Christmas?
Kinki Kids: Another Christmas
L'Arc~en~Ciel: Hurry Xmas
I Wish
Nakayama Yuma: Sayonara Merry Christmas
T.M. Revolution: Burnin' X'Mas
TOKIO: ding-dong
Utada Hikaru: Can't Wait 'Til Christmas
ZONE: Bokura wa Santa
For some other J-pop Christmas songs I know there is;
BREAKERZ: Winter Bell
Crystal Kay: Shining
Snowflake
KAT-TUN: White X'Mas
Nagase Miyu: Miraculous Night
Sexy Zone: Sexy Summer ni Yuki ga Furu