Port mirroring means you can only be a passive eavesdropper. Attacks like SSL mitm wouldn't work because you actually have to intercept and modify the traffic
SSL MITM still won't work unless you want it to be very noticeable or you have very substantial resources.
Port mirroring is enough to capture SSL traffic and to break weak SSL keys or if you have compromised the key of the destination services (w/ some caveats like no forward secrecy etc.)
And it doesn't prevents you from executing MITM attacks from upstream or just doing specific MITM attacks from within the TOR exit node later on.
But overall there is nothing you can do to ensure that your TOR exit node, your VPN gateway or even your ISP isn't reading your traffic other than to use encrypted tunnels everywhere and even then you are for the most part only moving the problem upstream.
