Good lookin' butt, almost looks like a nice glazed double-smoked ham!
If you want a bark (crust) you need to keep the surface dry for at least the final hour or so. For a really well developed bark, you can start the process in the fridge after rubbing...wrap in plastic wrap, place in a rimmed platter, bowl or cake pan to catch any leakage if it happens. Oh, never used mustard myself, and get great bark. Rest in the fridge for a couple days ahead of the smoke if you like, or overnight (8-12hrs). This resting time will allow the salts in the rub to begin to draw a bit of juices from the meat and the dry rub will soak them up and really tack up on the meat quite well. You can just dry rub and let it rest in open air while you get final preps ready on the smoker for 20 minutes or so, also. This helps to temper the meat a bit by letting it warm slightly just prior to hitting the smoker grates. During the smoke, your dry rub will also absorb more meat juices and some rendered fat as the meat cooks. This all combines to get crispy over longer smoke times, and with proper chamber temps and dry rub ingredients, will not scorch or burn. Speaking of scorching, added sugars in a butt or brisket rub is a bit risky due to the sugar's tendency to caramelize at lower temps, leading to scorched bark, and especially over longer cooking times such as with pork butts, pork picnic shoulders and beef brisket, just fyi.
The rub, wrap and rest in the fridge probably will yield the best bark for smoke chamber temps of 225-245*. I've also noticed that I get the most prominent bark in a charcoal rig vs propane, so chalk one up for the coal-burners. You can get a nice bark without the wrap and fridge rest also, but whatever you do, don't baste/mop/spray...it's all about getting everything on the surface dried-up, at least towards the end of the smoke. The pork shoulder cuts have enough interior fat and collagen to help them stay moist inside on their own, so to mop or baste is really not necessary, IMHO, and will kill your bark deader than a door-nail.
The mop/spray process you used for this butt would work well for ribs, btw, if you didn't want bark, but instead wanted a nice glaze, cuz that looks freekin' good from where I'm sitting!
If there's anything else we can help you with, just give a shout.
Edited by forluvofsmoke - 4/15/12 at 8:00pm