Looking at the Striations on the field I will say deterioration on the mint mark. IMHO What year is your Nickel?
What year is it? Have you done any research to match it to anything? Your images don't show much of anything at this point other than a worn die. Better lighting and focus are required.
http://varietyvista.com/05 JN RPMs/RPM Listings.htm Take a look here. If you don't find a match, then likely it's not a D/S. By now, I'd think the big ones are all attributed. However new ones get added from time to time. I personally don't see it.
I agree with @paddyman98. Based on those striations around the mint mark, it looks like die deterioration.
First Welcome! As a quick reply. Before ~1992 modern coins, ALL mint marks were punched into dies at the Philidelphia mint Die Room. Sometimes the die sinker would punch the Mint Mark in the die more than once. If he/she punched the die first with a "D" punch and then punched the die a second time with an "S" punch (or visa versa), then you get an over-mintmark (OMM) error. As to your specimen, without better images or data, it is too difficult to even guess.
Brightness may cause minor distortions. Pic's of the entire coin, obverse and reverse, would help considerably. Welcome to CT and good luck.
I'm not going to wait for better photos as I think the members that responded are correct. Just a worn die, and/or bad photos. Try posting Full Image and NOT Thumbnail photos. I have posted this before, but maybe you missed it. For the best results, you should post "in focus," FULL IMAGE photos (after you upload your photo, two buttons appear: Thumbnail and Full Image, click Full Image and your photo appears full size on your post and are easily enlarged by clicking on it). Photograph coins on neutral backgrounds like black, grey, or white. Crop out superfluous background so just the coin shows (you can use https://www.remove.bg for free), and post photos with correct orientation so members don't have to turn their computer in some awkward fashion to view it properly. While it isn’t always necessary, it is nice to show both the obverse and reverse, even if your question is just about one side. Members can often give more valuable information having both sides to evaluate. Add close ups of areas you have questions about and make your questions as definitive as possible so we know what you are asking for. And try to have the best lighting possible to show the most favorable photo of the coin. Hope this helps in the future. Good luck.
Wow, that is some stretch to get to a D over S, but I certainly would leave it to experts to discern the mint mark.