They are two separate issues, you don't look at the returns on SPY vs every currency.

If you are in USA and invest outside USA. Do you look at returns in USD or in nominal value of the market you invested in? Say there is hyperinflation where you invested. You should be extremely happy. After all the nominal value of your investment is massively up. Even if USD value is now fraction...

If you are outside the USA, then you absolutely do. Returns are denominated in the base currency but it doesn't paint the whole picture.