It would not. ARP is solely a protocol for LANs you are connected to. It cannot travel outside the network you are connected to. I must also point out that ARP poisoning does NOT work on websites for that very reason. ARP poisoning ONLY works for devices on your LAN.
However, mptcp includes some other vectors you could use. A SYN or ACK flood could be effective if they dont have a well configured server. If they have a badly outdated firewall then a Ping of Death style attack could affect it as well. There is also an "HTTP Attack" option, though I am unsure as to what that does.
