I assume you would have an early morning flight to SXM out of JFK (9:00am or earlier meaning you are arriving no later than 7:00am for an international flight) and if that's the case, going from LGA to JFK is no big deal. I regularly fly out of JFK on early morning flights and I pass LGA on the Grand Central Parkway to get to JFK. From my home in the Bronx to JFK for those early flights I never need more than 45 minutes and from the time I am passing LGA it's about another 20 minutes to JFK. I do this frequently and I have never missed a flight at JFK. Now if you have an evening flight, that could take more time due to traffic.

However, for peace of mind you might want to settle in right near JFK so there is no worry in the morning.