Its time for a quick PSA, this isn't even drone related. This can apply to just taking tourist photos on your phone, pictures of your food, family photos, FPV flight, Gopro action flicks. Whatever it is, if there's a lens involved, you should keep it clean to get the best image quality.
Your thumb, or any part of your body is NOT a way to clean your lens, your skin has oils on them, which just muddy up lenses even more. So use a micro fiber cloth and make sure its truly clean.