Well, it wouldn't be an error anyway, if it were a micro mintmark that would be a variety. I'm not aware of any for nickels. Your coin appears to be normal to me. Remember that the punches were changed once in a while, so don't compare the mintmark on 1981 nickels to, say, 1996 nickels - very likely that the mintmarks would be diffrerent sizes. Only compare the sizes and shapes of mintmarks to other coins from the same denomination and year.
The only mintmark size or shape differences I am aware of that are known on nickels would be:
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