To mustard or not mustard, the age old question I've seen in Q. I used mustard reflexively when I first got a smoker and started reading the stuff on the internet. I mean, it looked like quite the hack to me as a novice.
I stopped doing that and notice zero difference in the end results, so using it doesn't hurt anything, it may just be unnecessary mess. I've never had a problem getting raw meat to hold a rub, especially something as naturally tacky as a pork butt.
As far as spraying, that may be "a thing" that works. A lot of people do it. I think it depends on the method or the smoker used. With my little unimpressive MES30, and I use the water pan per the directions that come with the smoker (you can find many who say to never use a water pan), there is enough moisture that spritzing would probably just retard bark. If using some of the pro-level rigs people have and no added moisture otherwise; spritzing, mustard, all the whistles and bells we see some people do may make a real difference.
I'm no pro and dont have a lot of clout or gravitas here (notice the low post count and no fancy badges), so take my comments as such.