A good car cover with a soft inner liner will not scratch a car - the dirt on the car already, or the dirt inside the car cover could possibly contribute to light scratches, etc..
Depends on how rough or gentle you are with your handling of the material, and the quality of the material..
A cheap car cover - well you get what you pay for...
Please describe the car cover material you are going to be buying at Walmart----
Good car covers come in several types of materials, some that are totally weather proof and some that are lighter and allow moisture to dry out underneath them in most conditions... If it rains constantly, well, no breathe-able car cover can keep dry until it stops...
A good cover will actually fit the shape of the car better and will hold tightly to the body of the car..
Cheap covers will maybe fit pretty good, some not so good...
A good cover will also come with grommets in the middle so you can attach their plastic coated steel cable and throw it under the car to the other side and secure it to that grommet and thereby holding the car cover more securely and preventing it from being stolen easily..
A good car cover will have really great elastic - especially at both ends front and back, to help it stay secure on the car..
Any car cover and windy conditions are not good when installing or removing... In this condition, it is best to have someone on the other side to help, so it doesnt act like a sail or blow off the other side from where you are standing..
If you get one, let me know and I will tell you how to best install it and remove it by yourself..
I know these things because I have been using car covers for 45 years...

Dan F