Nobody here can really define what is "acceptable" or not for your situation. I think what you're really asking is whether it is a good idea or where it might be a problem.
The short answer is that it might be acceptable for a simple solution, but for anything complex it's a bad idea. Static classes make unit testing very difficult (or impossible). Instead, as mentioned in your comments, the better option is to define an interface that has GetSettings methods on it. You can implement it with a class (not static) which just wraps calls to ConfigurationManager.AppSettings as David L mentioned in comments.
Any class which requires access to settings should have this interface passed into the constructor. You can either use an IoC container or create a new instance of your class to pass in (depending again on complexity) but more importantly you can now write unit tests and pass in a class with custom settings for your unit tests.
