Will having a plastic bed cover only encourage my child to wet the bed more?

No. No normal child likes to wet their bed. Very few children wet their bed because of "laziness". Also very few children "couldn't care less", although they might give this impression. It is part of a natural defense mechanism, just as denying that bedwetting is a problem. A plastic cover will protect the matress and removal of the bed cover can be an additional goal for the child to acheive control.
Depends. Bedwetting is, in my opinion, about as stressful for the child as it is for the parent. So, the child would likely not see the plastic as an "excuse" to wet the bed.