If I'm not mistaken,JB's funeral dress had a high neck on it.(Deedee is good with things like this,am I right?) It was a used dressed she had never worn before,one which was to be worn in an upcoming pageant.I'm not saying Patsy particularly had that in mind while carrying out this crime,but the neck area isn't difficult to conceal,what with high collars and scarves and such,always being a possibility.
Of course, I haven't seen a photo of JB in her coffin, although it would not surprise me to learn that one exists. But most of the typical pageant dresses do have high necks. While girls wear several costumes at each pageant, most of the frilly (and expensive) dressy dresses have high necks and usually puffy sleeves, as well as the typical very full skirt, with lots of crinolines.
Also, JR allegedly put a scarf in the coffin with JB, don't know if he placed it on her neck or across her throat.
You'd be surprised what mortuary make-up can actually cover, but I do not think PR would have had her appear in the coffin with that mark on her.