Yes, Sunday afternoon is ok to visit SWF.
When you arrive in the afternoon, the park will be crowded.
Afternoon is like rush hour at any park. But lucky for you, SWF stays open until 10pm. The tired will leave, and the crowd will become berable as the night goes on. That's when you can get your rides on JTA and Kraken. I'd start the afternoon by seeing shows, get to each one 20 minutes before it starts. Then head to the hospitality house for some free beer samples, located in the back corner of the park.