The State of Palestine is. Hamas is more like a political party, so your question is like asking if the Conservative Party is a member of the ICC, it makes no sense.
Hamas is a party in the West Bank but in Gaza it is the government.
Being a political party and not a state, Hamas cannot enter into treaties, no more than the local chess club can enter into a treaty. Depending on the political processes of Palestine (of which I have no idea) it might be able to withdraw The State of Palestine from the Rome Statute, but it hasn't done that so far.