I have seen reports of this, but I've also seen other reports of the opposite. Some servers may experience some people who are "not paying" for their meal on the plan skimping on the tip. Some servers may get larger tips when some people on the plan up the meal "cost" by ordering the more expensive entrees plus desserts and soda for everyone rather than sharing dessert and drinking tap water. So, different servers may have different feelings about the plan.
However, if the server gives someone bad service because they're on the plan or not on the plan...they're a poor server and you could get bad service from them for any of a multitude reasons unrelated to the plan. For what it's worth, I have never yet received bad service at Disney dining and I have received outstanding service a couple of times whether on or off the plan (I've done both at a variety of restaurants).