In 1996, Kagan did give her opinion on the U.S. Supreme Court ruling in Seminole Tribe v. Florida, a major stepping stone in the Rehnquist Court’s federalism revolution. In a memo to then White House Deputy Chief of Staff Harold Ickes, she said the decision had “broad significance,” adding with prescience:
“The decision will doubtless stand in the way of at least some citizen suits brought to enforce federal law (as it barred the Seminoles’ own lawsuit). And the decision, especially when viewed together with the holding last year that Congress lacked authority to prohibit guns near schools, indicates a serious effort by a bare majority of the Court to reorient the balance of power between the federal government and the States. It is highly unlikely that this case will be the last one to pursue that states’-rights agenda.”
That’s the closest I’ve seen to Kagan actually giving her opinion on a matter of constitutional law, so I suppose it is significant in that sense. But even so, it’s pretty blase.