mb.io does not charge fees for crypto deposits. However, you will pay a network fee (gas fee) to the blockchain when sending from an external wallet — this is paid to the network, not mb.io, and is visible in your external wallet before you confirm.
Most cryptocurrencies have a minimum deposit amount, which varies by asset and network. To check the minimum: go to Deposit → Crypto, select your cryptocurrency and network — the minimum will be displayed on the deposit screen. Deposits below the minimum may not be credited.