Maxwell Lord proves Superman doesn't have high defenses against telepathy. Hence why if Wonder Woman didn't kill Lord he would have never let go of Superman.
We have evidence from more then one story showing you Superman doesn't have high defenses against telepathy. Darkseid has controlled him, Maxwell Lord has controlled him, Despero nearly took control of him, and more. Yes Superman will be taken down easily by Martian Manhunter with telepathy.
You're picking some pretty bad examples to 'show' that Superman doesn't have high defenses against telepathy.
Maxwell Lord, while not a full telepath but purely mind control, is an extremely powerful mind-controller.
Darkseid's telepathy is highly powerful, and as someone already pointed out, he didn't just use his telepathy to control Superman.
And Despero is a telepath who is even more powerful than the Martian Manhunter.
Using those three as examples of 'low mental defenses', would be like saying that <insert arbitrary Marvel character> has low mental defenses because Charles Xavier was able to affect their minds.
As Force pointed out, Superman does
actually have a very high level of mental defenses, in the form of his own will power, as well as his training in Torquasm-Vo, and to a lesser degree, Torquasm-Rao.
That said, the odds are in Martian Manhunter's favor, primarily due to his large variety of powers.