(I'm only speaking from my experience with mbuna's).
Typically, no, but the possibility exists.
Lip-locking is a way they test each other's strength and is used to determine dominance between two fish that are near-equals in that regard.
In rare instances, injuries can occur from this activity:
The most fierce lip-locking bout I've ever witnessed in my tank was between the alpha-male of the tank (a full grown 4" pseudotropheus demasoni) and a maturing male red zebra of equivalent size. The demasoni ruled the tank because it was fully matured and all of my other mbuna's were acquired as 2" juvies.
But when the red zebra approached 4" it decided it would try to take over and challenged the demasoni in an intense lip locking bout, sometimes breaking off to fight and then re-engaging. They eventually took the battle inside a hollow artificial rock decoration (the demasoni's key territory) where the fight continued for some minutes.
The demasoni then bolted out of the hollow cave, pursued by the red zebra which had its lips about ripped off!
Despite its lip injuries (which eventually completely healed), the red zebra chased the (apparently unscathed) demasoni for while and took over as alpha fish. I had never seen lip injuries like that before or since (this occured in 2005)...and he was the victor.