Is less quality cream used in salted butter?
There is no such thing as "lower quality cream" although there are variations in taste.

All milk has a slightly different water to fat ratio. In general, animal milk is 80% water, 5% protein and 5% fat.

Cream is the fatty part of milk skimmed off. Cream will typically be 35% fat (though in different cultures it varies from 20% to 75%). Cream also has some protein, and the rest being water

Farmers are paid for the milk solids (protein and fat), not by the volume of milk produced. Therefore quality milk is often referred to by how high the solids level is, not how flavoursome it is. Hygiene and storage conditions are usually government mandated, and follow international guidelines, especially if the manufacturer ever wants to export the product

Butter is cream churned to remove most of the water and protein. Butter in most countries must have 80% or more fat content to be called butter. The water and protein by-product is called whey, and is processed into other products

The 1% to 2% salt added is a preservative and a commonly desired flavouring, regardless of the "quality" of the cream.

You can buy unsalted butter, just keep it in the freezer for long term storage. Freezing butter has no noticeable effect on it.