If you can a) route the traffic through your proxy and b) generate a valid certificate for the target site that's accepted by web browsers, then yes, it's a mitm :)
Whilst a) is possible under some circumstances, b) should be impossible.
...unless you have somehow acquired a CA's private keys, or are in control of the clients' certificate stores.
The latter of which is very common in corporate environments where employee's machines are provisioned by the IT department and contain the company's private CA (which is useful for non-nefarious purposes such as signing certificates for internal services).