Probably. I'd be sure to get all the mustard out before I put any pepper plants in the ground. Some companion planting lists have peppers and mustards listed as enemies, so I wouldn't want to grow them side by side at the same time. Some mustards are allelopathic but since I don't grow a lot of mustard, I haven't paid attention to what plants don't like them. I usually just grow mustard in mixed beds with other brassicas and don't really worry about whatever succession plant follows them.