Market cap doesn't seem to be an appropriate metric to be used for comparing between currencies since how can you count precisely the market cap of the Latvian Lat in US dollars if not by using its exchange rate? The task of finding the market cap of the US dollar itself seems to be even more daunting. What are you going to count it in, in the same US dollars? I think a proper metric that accurately reflects the strength of a currency is the share of the world GDP (that should be correctly called GWP) which is denominated in that currency (i.e. total volume of goods and services produced by the country which emits that currency)...
Even if we take the GDP figures, we can see that the Latvian Lats is far behind the US Dollar. The American GDP is around $15 trillion right now, while that of Latvia is just around $32 billion. That means the the Latvian GDP is around 1/500 of that of the US GDP.
My point was not to show that the Latvian economy is somehow on par with the American one. I just tried to demonstrate that the comparison of market cap of a pair of currencies (i.e. total quantity of money in circulation denominated in some reference currency) is not much better than directly comparing the exchange rates...