I commend to your attention Michael Pollan's The Omnivore's Dilemma, in which he explores a considerable part of the history of fast food.

The short version, though, is that the food you'd have got in the 1950s would probably have been considerably better tasting than fast food today. I'm not familiar with the Sonic chain, but unless they're a very remarkable business, they can't help but be part of the same kind of commercialised food processes as McDonalds, Burger King, etc. I'm leaving KFC aside on that, because the presence of actual bones and meat texture in some of their food does indicate that it's a little less processed - maybe very little.

One of Pollan's major points is that the more processed food is, the less good it is for you, and in most cases, the less good it tastes. The level of taste is pushed back 'up' with condiments, salt, and so on, but if you get a plain meat patty from a fast food place in 2016, they're only barely edible, and you'd be hard put to identify which meat you're eating. In the 1950s, the industrial complex that goes from artificially-fed-steroid-boosted-antibiotic-drenched feedlot steer to burger was in its infancy, and there was still a reasonable chance that the local diner was using meat that was from reasonably naturally raised cattle. It was still processed, mind, and possibly supplied frozen to the business.

Cost-wise, fast food is about as cheap now as it has ever been, in terms of amount of food per percentage of income. However, McDonalds was very notable when it first opened in the 1940s because it was a 'sit down' restaurant to which working class people could afford to go reasonably regularly. Before that, you could buy similar food in many parts of the US from roadside stands or carts (or from baskets at rail stops in parts of the South more or less from the end of the Civil War), but not sit down inside to eat it.

However, one thing to bear in mind is that people's concept of what tastes good shifts over time. If people are used to and enjoy modern fast food, then they might find that the 1950s style version of the same is unfamiliar - probably a stronger taste of meat and bread, more distinct vegetable tastes from whatever tomatoes, pickles, and lettuce are applied, and less in the way of salt and soy. So saying that it would taste better is subjective!

Sources:

Michael Pollan, The Omnivore's Dilemma: A Natural History of Four Meals

Bill Carey, Fortunes, Fiddles & Fried Chicken: A Nashville Business History