every servlet can have a configuration object. when we say class Ball implements Bounceable, we can think this as that a ball has the property of bouncing. it can bounce. extending the analogy to GenericServlet implements ServletConfig we can say that a generic servlet has a configuration stuff associated with it, which is very much reasonable.
but i think we are deviating from the original topic , which is as to WHY generic servlet implements ServletConfig. the clever answer would be thats how the spec implementors thought of GenericServlet class which abstracts Servlet Functionality