That’s weird. I saw an achievement pop up for someone the other day. Maybe it was a generic PvP skin achieve?
There is a generic PvP skin achieve that is from gaining Honor Levels 10, 30, 50, and 80. The only other PvP appearance is the Challenging Artifact appearance tied to completing the Legion version of Mage Tower. This one specifically says "Win 10 rated battlegrounds after unlocking the appearance + Complete “(insert relevant quest here)” "