What’s wrong with owning a clothing line as president? As long as they’re not using their office to impact their business in any meaningful way, I have no issues at all with it. So before taking the oath of President, you should be required to demonstrate that you have removed yourself from any obligations or loyalties you may have (so appoint someone to take care of your company(s) and whatnot).
As long as decisions made by the company have zero relationship to the actions of the President, I’m fine with it. And Congress and the Supreme Court should have some checks against that (i.e. Congress should be able to set rules for the President through legislation, and the Supreme Court should be able to review executive actions and force divestment if there’s evidence of bias).