I don't have as many years judging under my belt as some here, but I have more than some. In my time as a judge, I have seen mustard sauce used twice -- once on chicken and once on pork.
The chicken was fantastic. It did very well at the table. The mustard flavor was there, but there was a lot going on in the sauce to compliment and tame the pure mustard flavor. Also, it was used very sparingly. Also, the entry was, in every other way, fantastic. The chicken looked very good, it was perfectly cooked, and it did very well at the table. Everyone seemed pleased to have something different -- that was done well.
At the last contest I judged, we had a pork entry with mustard sauce and it was everything that the above chicken entry was not. The sauce was terrible, and the pork was ugly, undercooked, and had a poor flavor overall.
Two entries isn't really enough for a decent sample size, but if there is anything to learn, I would say that -- just like anything you put in the box -- the entry has to be good. The only way you're going to know if it was the sauce's fault is if everything else is on point.
On the topic of having a personal dislike of certain flavors -- if you can't put your personal feelings aside, you shouldn't be judging. Disliking mustard is hugely different from someone turning in an entry with too much spice. Too much spice, that either overwhelms your palate or dominates the flavor of the meat, is a bad thing. Something as judges we specifically talk about as being a bad thing. Mustard, however, is a flavor profile choice, and if it is used with restraint and in balance, then it should be judged just like any other spice a cook uses.
Personally, I do not like fish, but I know what good fish tastes like. I know what the texture of tender, well-cooked fish and seafood should be. I can fairly judge fish and seafood. I prefer not to, but I have done so.
If I hear a judge at one of my tables say that they judged something down just because they don't like a particular spice, you can rest assured that I will be talking to the contest rep about that judge. It isn't fair to the cooks or the process.