everything depends on how market cap is measured.
you can only measure market cap when it is possible to directly convert the coin into dollars.
even if this is possible you cannot simply sum up what the current price tag is. you would have to estimate how the coins ecosystem reacts to big conversions.
this means you cannot simply sum up the coins in existence and multiply it by the average exchange rate.
another big point in terms of market cap is how people rely on services based on the coin.
if there exists a healthy ecosystem the coin can handle huge coin to dollar conversions simply because people also swap from dollar to coin.
to bring all this into a relation to archive some sort of "normalized" market cap is only as estimation possible.
in other words it means there does not exist anything like a marketcap of a coin which can be compared to another.