I'm sure the signatories would still consider their use to be a war crime. But at any rate, it's a clear escalation of weapons being provided.
A couple articles from Wikipedia:
https://en.m.wikipedia.org/wiki/Cluster_munition
https://en.m.wikipedia.org/wiki/International_humanitarian_law
It's a an escalation yes, and there's real human cost because it'll probably be impossible to get a 100% success rate on cleaning them up after the war, and thus there's a good chance that at some point civilians will die, this is probably why the US has been hesitant, but civilians are going to die anyway, this war is going to escalate (one way or another) anyway.
And no, they wouldn't consider it a war crime because Ukraine didn't sign up to not do it... That's not how it works.
Also yes, those Wikipedia links agree with me, I guess I should have posted them to show you your mistake instead of relying on my memory, thanks.
Edit: If you click though the cluster munitions page to the page of the treaty I was talking about there's this line "In response to U.S. lobbying, and also concerns raised by diplomats from Australia, Canada, Japan, the United Kingdom and others, the treaty includes a provision allowing signatory nations to cooperate militarily with non-signatory nations. This provision is designed to provide legal protections to the military personnel of signatory nations engaged in military operations with the U.S. or other non-signatory nations that might use cluster munitions.[25] David Miliband, who was Britain's foreign secretary under Labour, approved the use of a loophole to manoeuvre around the ban which allows the US to keep the munitions on British territory.[26]"
This isn't like some crime against humanity that everyone can agree should be against the rules of war and people should be punished for doing it even if they don't sign a treaty saying they won't. There's apparently a specific clause saying that signatories can continue to work with people that use cluster munitions... That's not the sort of thing you put into rules about absolute moral failures. They absolutely aren't going to turn around and start hating on Ukraine for doing it.