Taking off peoples cloth during torture is quite common, even nowadays. It's part of the humiliation process. You want to break people mentally and strip them of their dignity.
But everybody knows that Mashima likes to put more and more fanservice in FT manga so... I won't really use this as an excuse. Plus unfortunately Mashima is quite bad at expressing suffering or crude violence (just read Berserk already and see the difference for yourself..) so it looks more like an ecchi scene than a torture scene. Not that it's completely his fault since FT is a shonen, not a seinen.
I still like FT and this arc is far from a bad one, but this manga could be so much better. I would really like to see some of the important characters take a different path. To this day Jellal is the only one with a real "character arc"
Council of Magic
Traitor
Memory loss
Prison
Crime sorciere
New council of magic in the future?
Erza/Natsu/Lucy/Gray are just members of Fairy Tail from day 1 to chapter 365. Zero evolution ...
ps : Erza should loose some fat on her hip and her ass. plus her boobs are too big