There is no direct evidence that I can think of Fredo expressing his unhappiness- but that doesn't mean that it didn't happen. We do know that he was identified by Johny Ola as a weakness in Michael's regime, right? Wouldn't that be reason enough for Michael to consider the soft underbelly of his regime an issue worthy of his attention? Had Michael paid even the slightest attention to Fredo, Fredo might have been able to summon the loyalty to his family and report that he'd been approached. Armed with this information, Michael most likely would have foreseen Roth's moves and end game and avoid the fiasco in Cuba.