It's totally doable, but it is risky. In addition to the risk of flight delays into LGA, which tend to get worse the later in the day one gets, getting from the arrival gate at LGA to the taxi stand is a bit of a walk (I loved the old LGA, where the walk was three minutes, max), and of course there could be a wait for a taxi. (Someone is bound to pipe in and warn of the risks of traffic between LGA and JFK; yes that's a risk, but not nearly as significant as New Yorkers like to make it out to be.)
So the real questions are, what is your risk tolerance, and what is your Plan B if you miss the connection?