quick question, I don't have much experience with TOR(actually nothing), is it reasonable to build a mobile cryptocurrency exchange, which servers are behind TOR, so that's hard to attack for hackers ?
Tor is just an access network. You can create a service and make it available on any access network, whether exclusively or not. Operating it on Tor exclusively doesn't make it hacker-proof. You need to properly harden the server as well as all access points into your exchange (meaning your executables), and even then there could be vulnerabilities which can lead a hacker to compromise your service.