I played around with this to see what I could see, and I found something rather amazing. It may or may not apply to you.
When I change the sort criteria on a playlist in iTunes on the computer it, eventually, changes the sort order of the same playlist on my iPad. Sometimes it takes a few seconds (for short playlists), sometimes it seems to require that I clear the Music app from RAM and restart it.
I recently changed my Music setup to use iTunes Match on both the computer and all my iOS devices. Before that I was syncing a few playlists and albums using iTunes on the computer. I don't know if this behavior came with iTunes Match or before, because it never occurred to me to check it before.
Anyway, if you are syncing with iTunes, especially if you are using iTunes Match, try sorting your playlist in iTunes on the computer. Wait a while, or even clear and restart the Music app, then see if you playlist's sort order changes to the same.
Anyway, there is no way to change the sort order on the iPad itself, and I don't know what the default sort order is. I was trying to figure out the default sort order on the computer (by using various sorts and comparing) when I stumbled across this unexpected result; which prevents me from figuring out the default sort, ironically.