My apologies for taking so long to answer. It is possible that whatever is making your tree look sickly has also caused low apple production, but more often it is related to weather or other cultural issues. Some years there is a freeze when the trees are blooming, and it can kill all or most of the flowers. It doesn't sound like that happened for you this year, but we did have a very cool spring that was not good for pollinators. Despite the bloom, it may be that few of the flowers actually got pollinated.
If a tree is stressed, it may not have the energy to produce large numbers of fruits, and so it may abort most of them at a very small size. Did you by any chance note if there were many small fruits early on? Stress may have happened in the past - a dry summer without enough water, flooding in the winter, nearby construction that damaged roots.
To determine what might be making the leaves look sickly (and if that could be affecting fruit production), you can either send good photos, or simply take a sample (a branch with leaves in as many stages of sickliness as you can) in to your nearest OSU Extension Office.
I can only be of limited help on telling you how to send a photo, since I don't know what equipment and connections you have, but - if you can transfer a photo onto your computer (or whatever device you are using), put it someplace you are familiar with, even your desktop. Then you can post it here. In the box where you type your question (or answer to this), at the bottom, it says Drop files here or choose them . If you click choose them, navigate to the location where you put your photo, select it, and it will load.
I hope that helps.