This is a quote from a news journalist. When you call someone a "mobster" you are making an accusation, and this could result in you being sued for defamation or libel.
By saying "reputed mobster" the journalist is not claiming that Big Pussy Bonpensiero is a gangster, only claiming that others have said that Big Pussy Bonpensiero is a gangster. They are only saying that Big Pussy Bonpensiero has a reputation as a gangster.
They are doing this to make it less likely that Big Pussy Bonpensiero will sue them for defamation. It is like saying "the alleged gangster", or "the person that some people say is a gangster"