Is it unmannerly to "stab" food?

No, your fork should never be used as a scoop. What she means by "stabbing" is basically holding the fork upright--perpendicular to the plate--and stabbing down into the food. Spearing it, and in the case of salad, chasing down several pieces you want in your bite. Of course you are meant to pierce food with a fork. That's what it's for. But, you use your fork tines pointing down and mostly parallel to your plate and gently spear your morsels. If the food is difficult to pierce or seems to be elusive, you can "ist" it onto your fork with your knife or sometimes a piece of bread. Never fingers!