If the older leaves are yellow on a plant, I would suspect nitrogen first. Nitrogen is mobile in a plant.
Nutrients are moved inside a plant to where they are most needed. For example, a plant will try to supply more nutrients to its younger leaves than its older ones. So when nutrients are mobile, the lack of nutrients is first visible on older leaves. However, not all nutrients are equally mobile. When a less mobile nutrient is lacking, the younger leaves suffer because the nutrient does not move up to them but stays lower in the older leaves. Nitrogen, phosphorus, and potassium are mobile nutrients, while the others have varying degrees of mobility. This phenomenon is helpful in determining what nutrients a plant may be lacking.
Mint is a leafy plant which responds well to nitrogen. Roots may have been damaged in the transplant process and it will take time for the plant to regrow the fine feeder roots which supply the nitrogen to the plant. If the nitrogen deficiency is caused by root damage, the problem should resolve itself in a couple weeks.
If the problem is a deficiency in the soil, it won't resolve itself. I would add a spoon of miracle-gro all-purpose to a 2-liter bottle of water, then pour about half of that on the root zone. I would expect to see improvement in a day or two. If you don't want to use miracle-gro, you could soak fresh grass clippings in water until the water is green (overnight or so), then pour this water on the root zone. This is a nitrogen solution and not a complete fertilizer like miracle-gro, but I'd still expect to see some kind of improvement in the next day or so.
I suggest removing the extra dirt you applied, then adding fresh grass clippings around the mint, then covering the clippings with the extra dirt. This will trap the nitrogen in the ground which the mint can use. It will also add organic matter to the soil, add aeration, and fight dehydration. Every now and then you can add more grass and more soil. After a while of doing this, you'll have some really nice soil.