The operative word here is "σκια" (skia = shadow). While I fully agree that, strictly speaking, the addition of "mere" is unjustified in the NASB's translation of Col 2:17, it is not entirely without merit.
BDAG offers three shades (no pun intended) of meaning for the word "skia" all of which surround the central idea of a shadow. The third of these listed by BDAG says:
a mere representation of something real, shadow (from the sense of insubstantial aspect, eg, shades of the dead … there develops the
idea of a contrast between image and reality … a shadow of what is to
come Col 2:17, … Heb 8:5 …
Therefore, I presume that the NASB is emphasising that the OT ceremonial system amounted to nothing more than symbols of salvation and not actual salvation itself. That is, "mere" highlights the unreality of the symbol as opposed to the reality.
Despite this, it is not necessary as the meaning is apparent with or without it.