Yeah.
Many people form texture packs/mod packs for their own personal benefit.
As long as modifications are done clientside and aren't given an advantage in the eye of a staff member, they're okay.
For example: Script kiddies wanna change it so they can "hack" hats? So be it! It's not like they're actually modifying packets in order to spoof the server into believing that they are wearing a hat that they are in fact not.