Anytime is a good time to visit Pattaya. Many tourists start showing up when it gets cold back home (Germany, U.S. for instance) this is “high season!” High season means higher hotel prices and more crowds!
In general hotels and transportation is much cheaper in Pattaya than Phuket or Bangkok. Beach okay for sunbathing and walking, for beach type swimming go to nearby Jomtien.
I like September, it’s getting close to the end of the rainy season and the hotels are still at the lowest prices and fewer tourists around. By the way, the rain does not fall all day long and usually stops in an hour or so. Sun shines nice and bright for the better part of the day.