If the proxy also gives you certs to trust it can. Many corporate proxies do this (silently, as they control the PCs) and I've seen more than one academic campus where to access the wifi you have to accept a certificate.

On iOS this comes up after connecting to wifi as a reassuring, secure-seeming, official-looking page that pops up inviting you to accept the new certificate. Do so and boom, you're vulnerable to a MITM.