This does look like a disease, though improper cultural practices will exacerbate it. This looks most like anthracnose, a disease that may be caused by several different fungi. Was the plant on sale because it was already showing symptoms? Perhaps you can return it.
The first step is to remove the diseased leaves. They will remain unsightly and not recover. Hopefully there are still some leaves left...
Giving the plant the best culture possible will help it resist the disease, but there is no guarantee of a full abatement of symptoms. Your plant, a Fiddle-leaf Fig (Ficus lyrata), wants bright light and will do best in a sunny window. Growing it in shade will lead to its demise. It also wants regular water and good drainage. Water it, then let the soil half-dry before watering again. It should not be left to sit in a saucer of water. Also it should be fertilized on a regular basis with a complete fertilizer.
I would also re-pot the plant, or at very least take it out to inspect the root system. As a general rule, I re-pot almost every plant as soon as it arrives.
Fungicides can be used to control the disease, but they are not always effective, especially when applied without expertise. Decide for yourself if that route is worth it.