Direct answer
If you are foreign-domiciled, the USPTO requires representation by a U.S.-licensed attorney. If you are domiciled in the United States or its territories, an attorney is generally not required, although the USPTO strongly encourages applicants to consider hiring one.
Foreign-domiciled applicants
Foreign-domiciled trademark applicants and registrants must use a U.S.-licensed attorney for USPTO trademark matters. The rule applies based on domicile, not simply citizenship or where the business sells products.
U.S.-domiciled applicants
Applicants domiciled in the United States or its territories may represent themselves before the USPTO. Doing so means taking responsibility for filing requirements, legal strategy, deadlines and responses.
When legal advice becomes especially important
Likelihood-of-confusion refusals, descriptiveness refusals, ownership problems, complex specimens, consent agreements, oppositions and other substantive issues can require legal analysis.
Check who is actually providing legal services
Only qualified U.S.-licensed attorneys may represent applicants before the USPTO where representation is required. Ask for the attorney's identity and engagement terms when legal representation is part of a package.