Firstly, while the conversation started with "I'm looking for input", the manager has suddenly moved it into a push for delivery.
If the design was ready to ship, then that won't be an issue, but if all you're looking at is a mockup, or a slapped together stylesheet, etc, then what was an attempt an encouragement has just lumped more pressure on.
Also, the comment assumes that the designer thinks it's "done". The request for input could mean "this is the direction I'm going in, does that look right". Telling them that you think it's "ready to ship" still takes ownership from them. You've just moved from being the boss who provides 2 cents on everything to the boss that wants everything to be done right now without taking the time to do it right.
Much better to say "I think it's fantastic. Great work! Is it ready to ship, or do you have more to do on it?"