I believe what you are asking is how do you see the traffic over an HTTPS connection.
To see the traffic you have to install the mitmproxy certificate as a root certificate on the client, in your case the mobile device. After the client trusts the certificate, then mitmdump will be able to see the traffic.
Installing the mitmproxy certificate on the client will not open up traffic that uses certificate pinning. To do this you typically gain root access to the client and disable the systems default SSL certificate validation. Doing this disables all network security on the device, so I would not recommend doing it.