It depends entirely on which part of Jomtien you're in. The central beachfront and Jomtien Plaza area are lively, with restaurants, beach vendors and a steady flow of tourists and long-term residents alike -- comparable to a relaxed beach town rather than the nightlife-heavy parts of Central Pattaya.
Move a street or two back -- Second Road, Third Road, or the areas around Wat Boonkanjanaram -- and it gets noticeably quieter and more residential. Further south toward Na Jomtien, the pace slows down again considerably, with fewer bars and more villas and low-density condos.
If quiet matters more to you than beach proximity, we can point you toward the streets and buildings that tend to suit that better, rather than assuming beachfront is automatically what you want.