I think MTB is cool, but then I'm looking at it from the point of view of an avid fan. I am pretty certain that the rest of the world doesn't regard MTB as cool, (with perhaps the exception of modern DH and 4X).
However considering the retro era as stated in the question I think it was cooler then than now. I remember the jealously on peoples faces when I said I owned a Saracen MTB back in 1991 and that wasn't limited to fellow cyclists, "normal people" thought it was cool as well. My mates who rode "racers" were sick with lust for my off road ability!
From the mid 90's onwards though, I think attitudes had changed - (probably with the introduction of mass market MTB's like the Raleigh Activator) and suddenly BSO's were everywhere. From then on MTB was "uncool".
Still doesn't help me with voting though.....
I'm going with cool, because I still remember the smug feeling I had when for a brief period in my life, I owned the "best" bike.
a bit light on retro rides right now!
05 Cinelli estrada
93 Saracen Ltd Edition - In need of love
93 Ozone O2 E-stay (the wife's)
2013 Whyte 129-S