When a hasty or inexperienced electrician is installing the wiring, the circuit box is sometimes inset too far into the studs, which gives too much room between the wall plate and the box. This gap can cause broken wall plates when pressure is pressed on them, and wobbly sockets that give poor confidence in the condition of the wiring. It is perfectly fine to put a spacer between the outlet itself and the circuit box to give the proper fit.
Turn the power off to the circuit at the main power supply for the home. Take the wall plate off and then remove the outlet by unscrewing the "ears" from the circuit box. The ears are the metal pieces on each end of the outlet that have the holes for inserting the screws. If you are unfamiliar with screw sizing, which is a fairly obscure skill, then it is probably best to take mounting screws down to the local hardware store and ask for screws that are identical but about 1/4 inch longer.
Purchase about five washers for each screw also. Size them so they easily slip over the screw shank, but still fit closely. After you arrive back home, push the longer screw through one ear of the outlet and slide three or four washers over the screw shank. Screw the ear onto the circuit box and look to see if the outlet is moved forward enough for the wall plate to properly fit without wobbling.
Adjust the number of washers you use until the fit is correct. Unscrew the first screw so you can also push the second screw through its ear. Place the same number of washers on it as you did the first screw, and then screw both ears to the circuit box. Take care that none of the washers are touching any of the wiring in the circuit box. Put the wall plate back on and test for wobbliness. If everything is OK, turn the power back on.