Adding insulation to an attic floor assuming you have even a small amount (~ 6 inches) will not make an appreciable difference in Air-conditioning assuming you have adequate attic ventilation - i.e. ridge vents, soffit vents, etc. Hot air rises so the 140F/150F attic air will go out the top vent(s) and the 100F ambient air will replace it.
Adding insulation to an attic floor will make a large difference in heating.
If you are having Air conditioning issues, the first thing to do is call your HVAC person and get your system checked. Being just a little low on freon or slightly out of adjustment will cost you both efficiency (money) and the lowest possible temperature the system can deliver.
Most houses are designed to be able to hold a temperature in the mid 70s or maybe upper 70s in more southern climates for about 90% of the cooling season days. The rub is climate change is happening. This means there are more days above 90F vs the 1960s/70s and on those hottest days the temperature is higher.
Just last week one of the weather people at an Atlanta TV station said we had already experienced 3/4 the normal number of days above 90 over the last 100 years.
All this is to say, it is not out of the question the system you have may not be capable of cooling to say 75 in current weather. Depending on a lot of variables, the solution could be a simple as the next larger size or if ducting needs resizing a whole lot more. Hence my statement the first item is get your HVAC person involved.