It is possible but you will be cutting it VERY close and have a much greater chance of missing your flight.
I would go with Phil&Liz and book the 6 p.m. flight. It is better to grumble about having to sit in the airport for four or five hours rather than grumbling about missing your flight.
One thing I have to differ from Phil&Liz about is that FLL is much faster than Miami. FLL is only a few minutes from the port while MIA is at least a 20 - 40 minute cab/bus ride.
If you must take the early flight make sure the ship knows that you need to get off early. It is possible to do self-disembarkation and be one of the first off the ship. This is fine if you only have a couple of bags and can easily handle your own luggage.
One other note: You will have to wait until the Carnival transfer bus is full before they will leave for the airport. You could save a few minutes by grabbing a cab. It will be about a $25 cab ride from the port to MIA.