TTT2's customs don't let you customize individual parts like hands, ankles, upper body, lower body, arms, etc. Tekken 6 allowed you to be very creative with each design.
Tekken 6 also had predetermined colors and a large variety of them BUT thankfully had nothing that was disgustingly obnoxious.
Tekken 6's customs were also character specific and reflected the design of the character:
It wasn't random nonsense like Heihachi with a weird T-Shirt and leather pants.
But most of all, the Tekken 6 designs had actual effort put into them. Their clothes shared the cloth effects of the default designs while looking at TTT2's letterman jackets and such that would just stick to the character like some PS1 asset.
The thing is TTT2 gets a free pass because we understood they had to make concessions to let 4 characters be on the screen while Tekken 6 only had to worry about 2.
Tekken 7 doesn't have that same limitation so going with TTT2's primitive, obnoxious, and ugly system rathar than Tekken 6's that was the greatest system in all of FG's is a backward decision.