Note that S&P 500 has been pretty tepid since 2015, and if you bought your 457 index funds post October 2014, your returns will be none/minimal.
I can't comment on whether it is financially savvy to pay off mortgage or continue contributing to 457 without further information, but note, also that it doesn't have to be an all or nothing deal. You can contribute may be $9k to your 457 and divert $9k to your mortgage. Not sure if it will help you, but just a possibility.
On a separate note, an expense ratio of 0.42% seems a lot, especially if it does nothing more than track the S&P 500 index. Those funds shouldn't cost that much. Hopefully your 457 gives you a few different funds, and hopefully there are at least one or two "Spartan" or "Admiral" type funds (Fidelity and Vanguard respectively).