I'm against it for two reasons.
1 - We are getting too many outfits to catch up. I think that CIP should focus on new and inovative way to change cosmetics rather than the usual outfit + mount.
2 - That's a too simple use for achievements. Achievements are the best mensure to tell if a character is getting everything from the game rather than the simple power level. You get achievements for completing quests, outfits, taming and many more. Maybe use it for the second promotion or class advance in the future would be more valuable.