I actually think that restaurants are a good comparative tool. "Dining" consists of more than just the food. The service, decor, ambiance, plating, china and cutlery all make it a dining experience.
With this information you can do a reasonable comparison to cruise lines.
However, you can't ignore the luxury lines such as Regent, Seabourn, Silversea, Crystal for those with high expectations of food and service.
BTW: I do consider myself somewhat of a foodie and I believe Olive Garden can often get a bad rap. If you know what to expect at Olive Garden and don't believe the hype that it is "authentic Italian cuisine" you can have a pretty decent and tasty meal. I personally have had better food at an Olive Garden than at a couple of highly rated, "fine dining", restaurants. It's the same with Carnival. If you know what you are getting you can have a great cruise experience that may be more "enjoyable" than an upscale cruise.