Cotto is 6-4 since his greatest win against Mosley so it's natural that people would consider him 'post-prime'.
Also, talking about a guy being past his prime 'skillwise' in boxing is kind of meaningless. Skill in boxing is inextricably linked to physicality and as such, one depends on the other. A guy throwing a technically delightful left hook doesn't mean much if he can't throw it with explosive power.
Physically, Cotto doesn't look to the be fighter he once was, even if in pure boxing terms he's still very good.