Happy Town is an anime-exclusive location in Johto, more specifically near Cherrygrove City and Violet City. The town's appearance is reminiscent of a small neighborhood, with houses with small lawns covering most of the town.
In the anime, Ash and Co. stopped in Happy Town shortly before entering Violet City. When they entered the town's Pokémon Center, they found a Blissey instead of a Chansey working there alongside Nurse Joy, and Jessie, while following Ash and Co., recognizes Blissey and explains to James and Meowth about her past in nursing school alongside the Blissey, a Chansey back then.