Is it okay for oil to smoke in stir-fry?
Yes, there's smoke coming off the oil, but it's not heavy, black smoke.

If you're frying things, when I see that, it's a sign to either get the pan off the heat, or to get food into the pan to cool it down.

When you're stir-frying, you want very high heat, so you're often working right at the smoke point.  Whereas, for sautéing, I always learned to put the food in when it shimmers, but before it starts to smoke.

So, should you let the oil smoke?  Generally, no, but it's okay if you use it as an indicator for when to put in food if you're quick about it.

update: To answer the culinary questions in your comment : 


The oil does denature, but it's not all denaturing at once, just like when you boil water, it doesn't all evaporate at once.  Or if you burn an item, the whole thing doesn't instantly turn to ash.  It's not a boolean (yes/no) answer, it's a question of the degree to which it's denatured. Letting it smoke so you know when to put in the food isn't the same as if you let it sit there an smoke for 5 minutes because you don't have your food ready.
Moist food will cool off the oil because the water will turn to steam, resulting in evaporative cooling.  This process ends up sucking energy (heat) out of the oil in the process.  Flipping the food repeatedly (as you would with a sauté and some people do with a stirfry) can also cool down the pan, as you have more heat transfering to the air (more surface exposed while it's being flipped), and you typically lift the pan off the burner so there isn't as much new heat entering the system.