Michael Collins was not a great leader

Collins never won us independence, all the Anglo Irish treaty gained was 26 county independence from the United Kingdom while still remaining part of the British empire, there was no guarantee of any independence it was simply the bare luck of future events that made Britain let 26 counties of Ireland be free. It was around 20 years after Collins death when the 26 counties of Ireland actually gained independence

Britain used the age old tactics of divide and conquer to end the war of independence, they gave some of the big players like Michael Collins big positions of power and turned them against the majority of the IRA to fight their war for them. The vast majority of the IRA were completely against the treaty. Collins took guns and advice from the British and fought the war against the IRA himself.

Another thing is that he was responsible for the deaths of many innocent people. His favorite targets were off duty police officers, Irish men just doing their job trying to earn a living. Collins was against freedom of the press, he used to send the boys round to attack any media outlets who talked bad about the IRA.