I did this precise trip yesterday, albeit in the other direction! Late morning, Piraeus to ATH took 50 minutes with a crazy taxi driver, 55€, bit of traffic, some stop-start but not too bad. Getting off the ship at Piraeus took a good 30 minutes however, which led to me taking the taxi rather than the bus/metro, That turned out not to be necessary since my flight was delayed a bit (and was a mid haul 321 so I was very happy!).

However you are in the rush hour, so I can see it may stretch to 90 minutes if you were unlucky. So personally I think it's a bit tight unless HBO. Fly LHR-JMK and ferry from there?