You don’t need to return a JsonResult because its job is to serialize an object into JSON string. You already have the JSON string, so just return it in a ContentResult and specify the correct content type:
string json = //get some json from your DB
return new ContentResult { Content = json, ContentType = "application/json" };
Remember that your MVC action methods should all have ActionResult as a return type, so you can return ContentResult just as easily as JsonResult.