A senior candidate has a serious, revenue-generating side business, maybe in a competing product area? Yeah, that seems like cause for concern: this candidate's potentially-dropped cycles working on their business could mean a significant loss in team productivity, we might worry that they're on the verge of quitting to focus on their own product, whatever.
An intern candidate has a side hustle? Awesome, sounds like they're not totally clueless, likely they'll be able to ramp up quicker than someone with less experience. Ideally they put it on the back burner for the internship duration but nothing is going to collapse if they're not functioning at 100%. I feel like it's a much bigger risk that you hire someone with a weaker resume and get nothing from them even when they try their best.