Are you on one itinerary or two?
If one, then it's a simple question of whether you've got a valid connection. It's international to international, so you would not need to clear immigration and would obtain your onward boarding pass in SIN. BKK is a pretty good airport and, even if you had to get from one extremity to the other, I would have thought you would be OK.
If two itineraries, then I would say it's not enough time (especially if you have to clear immigration). You should be able to check your luggage all the way through, however.
It is one itinerary. Also I am considering my inward journey into BKK (arriving on LX in F) and changing over to TG (in C) for onward journey to Phuket. In that case, would I have to clear immigration at BKK or HKT? I will have 1.5 hr layover.