Definitely NOT. If is sometime possible to have an outdoor unit slightly UNDER the ton rating of the indoor evapoarator. It is NEVER ok to have the outdoor unit oversized. Not only mismatch is the evaporator unable to keep up with the liquid being sent to it, but now your condensor coil is wrong also!
Was the evaporator installed at the same time as the condenser?
At 15 years, it might just be time for a new system.
Has a good HVAC tech evaluated this system to see if it is truly undersized? A good inspection would include all the duct work in the attic to make sure you are well insulated and have no leaks.