What were Attlee's mistakes?
The two biggest, objective blunders I can think of were the
failure to prepare for the 1947 winter and the infamous
Tanganyika groundnut scheme. We also withdrew from the Palestinian mandate and India during his term; which obviously caused a lot of problems down the line, but Labour can't really be blamed too much for those disasters - Palestine was an inherited disaster zone with no real solution between the feuding Jews and Arabs, and the blame for the infamously botched partition of India is shared between all sorts of people.
On a less objective level you can criticise him from the Left or the Right (depending on your personal opinions). Right-wingers would obviously oppose his nationalisation of industry, the creation of the NHS, the expansion of welfare etc. From the Left, you could (if you wanted to) criticise his foreign policy (largely Bevin's department) which essentially consolidated the strong Anglo-American alliance that exists to this day, the slow dismantling of the African portions of the Empire, the creation of the nuclear deterrent, the use of the military to end the Communist-supported 1947 dockers' strike etc.