MetaMask chrome Extension is best suited for those currencies that can be traded on the Ethereum blockchain or you can say that ERC-20 tokens. And if you think that you can only use the ERC-20 tokens then you are wrong. Because the users who want to use tokens other than ERC will have to trade their ERC-20 tokens in exchange for the tokens you want to trade. Hence we can say that trading other crypto assets that are not ERC-20 coins is possible.