Obviously, when we talk about "dress" sneakers, we don't mean "dress" as in something you'd wear to a formal occasion or with a suit; we mean it in the sense of "dressing up" an outfit in a relative way.
"Nice" sneakers, which of course exist (at least, they exist in the world of people who don't use words like "shan't" or "horrid" - "normal" people, let's call them), simply are less over-designed than functional trainers or running shoes, and may be made of nicer materials (leather, a heavy canvas, etc) in more muted colors (browns and tans and so on.)
So, for example, a pair of vintage-inspired suede SeaVees Hermosas is "nicer" than a pair of Converse high-tops, or a running shoe. They might "dress up" a casual outfit to a greater degree than a technical athletic shoe would.
So I would say "dress sneakers" are less an oxymoron than a valid concept misadjectivated.
"Hon? Have you seen my *nice* sneakers?"
"The cognac suede ones? Yes, I put them where *you're* supposed to put them. In the shoe closet."
"Oh. Okay."
DH