Yes, but if you want to get that technical, the correct form depends on the gender of the latin word for "parable". The two options are parabola (which has the "parable" meaning only in late Latin, I think) and fabula ( == fable, e.g. Aesop's). Luckily, both are feminine so non gratae is correct.

Although you can make an argument that the English word "parables" has the neuter gender and so it still should be non grata :-)