I do not like mustard. Is there anything I can substitute? Would mayo work? Thank you!

The purpose of the mustard is to allow the rub to adhere to the pork butt. Otherwise it falls off the meat and you use a lot more rub. You will not taste it, so use the cheapest prepared yellow mustard you can find.

You are not going to get a mustard taste from a tablespoon of mustard spread over a four pound roast. Be adventurous and try it. If you really don’t want to, then you might mix a tablespoon of vinegar with the sugar and syrup. That would balance the sweetness.

The mustard is what counterbalances the sweet ingredients in the recipe. Mayo wouldn't be the proper substitution here. Instead, I'd look for a recipe that calls for a different flavor profile. A google search for overnight roast pork turned up one from Jamie Oliver. I'm sure there are others too.