No, not always. Whether you need ESTA for Guam or Saipan depends on your nationality and which travel authorization you are eligible to use.
If you are eligible for the U.S. Visa Waiver Program (VWP), you can travel to Guam or Saipan with an approved ESTA.
However, eligible travellers may also visit Guam or Saipan under the separate Guam-CNMI Visa Waiver Program without ESTA. In this case, they must obtain an approved G-CNMI Electronic Travel Authorization (eTA) before travel.
ESTA and the G-CNMI eTA are separate travel authorizations with different eligibility requirements. Before travelling, check which program applies to you and make sure you have the required authorization.