Spoonhead is a fault in a show Betta of any fin type. Fighters are the exception; the upturned mouth is said to be devastating.
The body line should be one smooth hyperbolic curve from nose to peduncle. Any dents, radical differences in curvature, flat spots or recurve (spoonhead) are faults. It should look like it was drawn in one smooth line with a compass or French curve.
Mark, the HM (or SDet) in your avatar has a good, but not exceptional, topline.
There's a wonderful drawing around here that shows just that. But I lost my URLs and I'm too lazy to find it just now.