Perhaps he wanted to make her his mistress ritualistically and wanted her dressed for the occasion, using her as a pawn.

He always seemed weirdly fond of her with his scheming, like when he says "You've been chosen, Sonya. Much to my delight".

^ I know he's dastardly but why else would he put his hands on a women? Never seen him take a female's soul (outside of the games and unsure if he took Li Mei's soul in her MKDA ending), seems kind of LGBT.

Seems like standard trope.

It could be a nod to Big Trouble in Little Chinatown, where Lo Pan specifically needed a blonde woman with green eyes.

Shang seemed interested in her. Could be for a number of reasons. Produce an heir, and soul transfer into them. Or could genuinely like his heir, and proudly teach them all his dark arts.

Ka-Tra

Because Kitana would never give him any... and the only way to get a woman to bang an old demon sorcerer is to challenge them in Mortal Kombat.

Seriously, it was supposed to be a nod to Sonya being captured prior to the events of MK2 and chained in Kanh's Arena. Why in a dress? Because sex sells. Period. Look at all the stupidities Conquest did in order to keep you little horn-dog teenagers watching that god awful series.

Never seen Sonya as the raunchy type, like Mileena was at times, so dressing her like this must've been done by force but you can't be assured if this was done against her will, if she was talked in to it. I know Shao Kahn wanted the Earthrealmers dead but she and Kano was spared and handed over to Shao Kahn from Shang Tsung and handcuffed. Maybe there's more to gain with them alive than departed, Kano being a criminal and that.

Yes but Omegis was known for courting with Shang Tsung, who was more attractive than Kitana but I get Omegis had unnatural beauty. Maybe the power he has turns some women on.

What was the other dress she wore as she was leaving the Outworld with Liu Kang, Johnny, and Kitana?

Dress? WHAT DRESS?
Clearly they beat up a "Shadow Priest" and stole his garments.

^^ All the above statements referring to "sex sells" are accurate. It's a bondage / slave thing, once someone is your slave you can make them do anything so long as you hold the power. Why specifically a brown, leather dress and not torn garments? Have no clue and have less care.

I wonder how she got into it?