The smudge will not affect the sound or shorten the lifespan of the guitar.
Having your forearm draped across the top when you play DOES affect the tone, because it keeps the top from vibrating as fully as it might. (Try playing an E chord with your forearm on the top, then lift your arm while the chord is still ringing, to hear whether your tone's being inhibited by your arm - on some guitars this won't make a difference, but on most there's an immediate and audible difference in the tone, with a fuller sound once your forearm is off the top.)
But, no, your perspiration will not affect the guitar in any way other than cosmetically.
Hope this helps.
Wade Hampton Miller
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