The question about the dress means looking at Hasi, and looking at Hasi means looking at Hasi. Mitchell is by no means easily flustered by women but its not like he can pretend she's not attractive. He's already admitted as much in their earlier conversation.

"I can tell you picked it with a purpose in mind," is what he finally settles on. Along with, "I don't think this place is really my scene."

It could be. There's something about Gutters that speaks to him in a way that Hasi is perfectly fine with accepting, but which Mitchell has spent the last several decades trying to deny as much as possible. He was always a little bit showy for his own good.