According to Wikipedia:
The technicians of the RoboCop Program, led by executive Bob Morton,
used what was left of Murphy's face and portions of his cerebrum and
cerebellum and applied them to a cybernetic body, in effect,
resurrecting Alex Murphy as RoboCop.
His original face was probably preserved and partially exposed by the visor to "humanize" him so that the public would trust and accept him better. Further, under the Body Structure heading:
When the visor is removed, only
Murphy's face (which is grafted onto a completely mechanical skull)
from the top of the neck up is exposed.
RoboCop implies that only Murphy's face and brain was used in the
construction of RoboCop, as Morton states that "total body prosthesis"
was an agreed-upon parameter. It is unclear in the first two films
whether or not RoboCop's human face is merely a replica of Murphy's,
as it contains a scar where Boddicker shot him in the head, though he
tells Murphy's wife, in RoboCop 2, that "they made this to honor him."
After touching it, she says, "it's cold."
In RoboCop 3, Dr. Marie Lazarus, RoboCop's chief technician, stated
that Murphy's face was indeed transplanted onto a mechanical skull,
and that it is not a replica.
The scar from the gunshot wound Murphy sustained can be seen when RoboCop has his visor off, further suggesting his face is not a replica, but in fact his original organic one:
So, presumably, his organic face could still take damage, but there is a metal skull beneath, so the damage would be purely cosmetic. I believe the scene you are pointing out occurred before RoboCop was able to remember his prior life as Murphy, and he was therefore still cold and logical. Logic would dictate that cosmetic damage is not something to be worried much about, which would explain the indifference to sustaining damage to the lower part of his face. I imagine once Murphy fully "reawakened" and remembered who he was, he might not be so cavalier with the last remaining vestiges of his organic self.