Recap - OP is looking for best way from Marriott Marguis @Times Square to LGA on a Tuesday afternoon for a 5:55PM departure, US Air @Terminal C

Concur with others that 2 hours might be a notch too close even with only carryons - TSA checkpoint/security can sometimes be long & bad, unpredictable - we always allow extra time. I would leave by 3:30 PM & no later than 3:45PM to plan for arrival landside between 4:45-5:15 PM.

NY Mets are away next week but playing at Citifield the week after, and whether it's a day game or evening one, subways & buses will be more crowded than usual when that happen - Willets Point Station might not be a good idea via the #7 subway (express or local) out of Times Square. On Game Days, it's packed with fans & very crowded, many arriving early and traffic on local streets are often a mess with traffic agents out in full force.

I would go north on Broadway to 49th Street, downstairs to catch the N or Q subway to Ditmas Blvd-Astoria, then to street level to wait for the next M60 Bus to LGA. Travel time will be about the same or just slightly longer than the #7 per MTA Trip Planner - about 1 hour in total. (Google's estimated 38 minutes are too idealistic ... )

It's much easier to find a taxi or car service at/near the Astoria station and almost no chance for hauling one at Willets Point - in case OP is running late/behind schedule - although traffic on the Grand Central Parkway toward LGA will not be all that smooth with the PM rush already underway in the afternoon.

And, of course, mother nature gets its say on the weather & on mass transit & commute, in addition to flight scheduling. Better to leave earlier than necessary, get some bonus time in the club lounge and then cruise easy to the boarding gate - than to sweat it out & watching the clock. That's just me, YMMV - as always.