I was not happy all summer with the air conditioner in my 1989 Oldsmobile Cutlass Calais. I brought it in for this problem and was told it was OK. The mechanic said the pressures were within specifications. I've had this car for a long time and I know it was running much colder until recently.
If the temperature difference from your A/C is that noticeable, as opposed to previous hot summer days, then there is definitely a problem that warrants repair. Exactly what the problem could be raises a lot of possibilities.
When your mechanic checked it out did he mention "Duct Temperature"? This is something you can check yourself if you have an accurate thermometer. First, put the thermometer in a center A/C duct outlet and then close all the others. Next, with the engine running, put the controls to "MAX A/C," this closes the outside air duct and recirculates the passenger compartment air to give you the optimum chill. Put the fan speed on "Low" with the temperature control all the way cold and let it run for about fifteen minutes. This will get your output temp as low as your A/C system, in its present condition, is gonna get it. We're looking for close to 40-45 degrees with the weather in the 80's or 90's. But it will vary with humidity also.
I'll presume that your readings were higher than they should be so let's run down a list of possible causes. A really common item is a refrigerant leak. The leak must be located with a special leak detector, repaired, and then the system must be evacuated and recharged with Freon (R-12) or R134a. The leak can be anywhere in the system: evaporator, condenser, receiver-dryer, compressor or any of the hoses, lines and fittings.
Other causes of insufficient cooling may be a bad compressor, a contaminated receiver dryer, a faulty expansion valve or an electrical problem involving the A/C compressor or coolant fan operation. It could even be something as simple as an adjustment of the temperature control cable.
Sometimes it's tough to accurately determine exactly how much refrigerant is in the system just by reading the pressure gauges. So perhaps you are a little low and the mechanic didn't catch it, but if he had seen a poor duct temperature he should have known there is a problem regardless of the pressure readings.
Start with the thermometer and take it from there. If you're uncomfortable with the first mechanic's answer, take the car elsewhere.