Rather than focusing on accountability as the starting point, I would suggest building tooling and visibility so that cloud costs are visible across all layers including application and infra. Once you have this, accountability becomes easier.

Each application team should be able to view the total cost of running their service - and thus be held accountable to reduce costs when necessary.

Without data you are running blind. Cost optimization cannot be solved by a standalone team - it has to be owned by everyone.

Source: Personal experience reducing cloud costs in a slightly smaller team.