It looks like environmental damage. Dimes weigh 2.27 grams. If it was missing a clad layer (usually 1 side only) it would weigh more in the 1.85 1.9g range. Missing clad layer would still have some luster, yours is just rusted/oxidized, probably spent some time in the ground/ metal detector find.
Alas, that is just discoloration due to environmental damage. Here's a dime with a missing clad layer.
And this one ? Can't really appreciate it in the photo but in half of the left side its different and shiny. Thanks