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.
possible? yes. reasonable? maybe, depends on your situation. you can use tor to hide the server of every website which you host but usually you use tor only if you have a reason why you want to be anonymous (or want improve the anonymity of your user). tor is no hacking-protection.