I have noticed that there are a few animation movies/TV series on IMCDB. Does this mean that any animated movie that has cars that resemble real ones can be added? Because (mostly in King of the Hill) many are really poorly drawn and are hard to identify. But movies like Toy Story and Cars are very identifiable.

Yes, we usually list animation movies when cars can be identified.

I don't remember the Toy Story vehicles being easy to identify [with the exception of the Toyota which really isn't a Toyota (check the owner's manual in Toy Story 2)].