You should have checked the state of the stroller when they gave it back to you at the airport. However, you did not. OS does not have to pay anything and the words you sent them exclude any possibility of a goodwill gesture.

Probably best to stop whining and chalk this one up to experience. Live and learn.

PS: As others posted before me, the cargo hold is known to be a dirty place. Thus, stains on checked items do not constitute grounds for compensation. You should have placed your stroller in a waterproof protection bag before gate-checking it.