The UN is not here to enforce anything. It’s a forum for nations to talk to each other, in the hopes that a peaceful resolution can be reached. They condemn things in the hopes that this soft pressure on heads-of-state makes them reconsider their actions.
Unexploded cluster bombs will be discovered in Ukrainian dirt for generations to come — probably by farmers and children. The UN has to condemn it.
UN is irrelevant in some sense I guess but that depends on what you expect them to be. If they could enforce things everyone would leave at once. It's a place where countries can talk to eachother that might otherwise not. And that is at least worth something.