Filler enhancement for the lips is very artistic and definitely requires more skill than say, filling of the nasolabial folds. There are subtle differences from lip to lip, and what may look great on one face may look awkward on another.
After treatment, one side of the lip can look fuller than the other because there actually may be more filler injected on that side.
Or, if the doctor performed a nerve block with anesthesia, the bolus of anesthesia may have been more on one side than the other.
Or, it may seem asymmetric because of the anesthesia causing more temporary paralysis of movement on one side than the other.
The best thing to do is let everything settle and then after 1 week reasess for symmetry. Restylane is soft, and the doctor can "mold" it so it is not lumpy or bumpy.
