The Weeping Angels had a unique and nearly perfect defense mechanism: quantum-locking, which caused them to turn into harmless stone when being observed. - source
Since their transformation into stone is a defense mechanism, it is likely that the sharp teeth are simply part of this defense mechanism as their prey would be frightened by such a sight and therefore be less likely to attack.
We all know they cover their faces to avoid trapping each other in petrified form by looking at one another. Notice that covering their face initially also gives them a non-threatening appearance. However, if they're threatened or they're hunting prey and the prey looks at them, they assume a scary appearance while quantum-locked as a defense mechanism. Also, something frightening and scary is more likely to make you look away or blink, allowing the Weeping Angels to advance.
Many real life creatures put on a scary display to frighten predators. For example, the Owl Butterfly mimics the scary face of an owl (even though the butterfly is much less powerful than an owl):