It’s not a great idea to fill the R134 refrigerant without knowing how much you are putting into the system. Overfilling is often times worse than under filling - if there is too much R134, the liquid/gas cycle might be inefficient, resulting in inability to cool, the compressor work has too hard and premature failure.
Your best bet would be to have the refrigerant high and low pressures tested with an AC test manifold. Any competent AC service location should be able to do this.
That being said, the first thing to check is to see if he compressor clutch is engaging when the AC is turned on. You should be able to hear the clutch click and see the outer plate on the compressor spinning with the belt when the AC is turned on. If it’s not turning on, the system might “think” there is insufficient pressure and a check of the pressure switch would be the next place to look.
Next, definitely make sure your engine coolant is properly filled and there are no bubbles in the system. A bubble caught in the heater core can result in different temperatures from different vents. I worked on a vehicle a few years ago that blew hot air out of one vent and cold out of another. It turned out to be a clogged heater core. Hopefully that’s not the case with your Smart. In another case I worked on with hot and cold simultaneously, a replacement thermostat remedied the issue.
Another thing to check are the motor actuators for the blend and temper doors. They are mounted on the sides on the heater/AC body. If they get stuck, the temperature might not change or may change less than what’s being directed by the temperature control.
If you system has been exposed to ambient pressures, moisture can develop in the system that can affect the ability cool. If this happens, evacuating all of the refrigerant and extracting the moisture is done using a vacuum pump. In some cases, an internal desiccant, typically a cartridge inside the condenser, needs to be replaced.
The Smart also has a temperature sensor mounted above the passenger footwell. There have been cases when the wires to this sensor have been damaged or the sensor itself is faulty.
I hope this helps.