Even though it was paid for by a private party, I agree.
Whether or not it technically qualifies as a First Amendment violation, it can still be taken as an open endorsement of one religion over another, which is not okay to do for an entity that's supposed to remain absolutely secular.
I don't much care for it (the monument at the Capitol), but what religion does it endorse? I know Christians want it there, but the commandments themselves are present in at least two religions (Judaism and also Christianity, which the commandments predate) and their general tenants transcend countless societies and religions. The specific monument in question actually has Stars of David on it. I don't believe it contains a cross.
This gets a big "meh" from me.