With the large mpls ping in process, the probability of hitting a time when MPLS is in use to process PATH/RESV (high proirity traffic) etc is more. Considering that most of the processing for ping reply happensin the control plane by the MPLS task while it also has to do other priority processings (rsvp path messages etc). The pick (max delay) response of mpls ping can go high. In the below output, the average reply came at 1 ms while the pick delay was 57 ms.
Success rate is 100 percent (500/500), round-trip min/avg/max=0/1/57 ms.