I have no problem with guests preferring bland food but snotty remarks and facial expressions are just plain bad manners and should not be tolerated.  Have them over as infrequently as possible, serve the boring stuff and have other dishes also for those who prefer them.  The  bland stuff takes no time to prepare and you can spend you time on the other dishes.  If the in-laws start to make derogatory remarks about the other dishes, look them in the eyes, smile and say ' I made an effort to prepare food you would enjoy and food the rest of us will enjoy.   Please do not spoil this dinner with unpleasant remarks about the food you are not expected to eat'.  Repeat when necessary.
I suspect that these people have a bit of an inferiority complex (about more than food) and making unpleasant remarks is their way of bringing you down to their level.