The environment - cold water from being in a mountain lake, dehydration from dry air at altitude - may generate the feeling that it's easier to get narced.
From a purely partial pressure point of view, diving at altitude actually would delay symptoms a bit, due to reduced surface pressure. For example, surface pressure at Lake Tahoe is 0.8 ATA, so this would push onset of narcosis ( 0.2 x 34 = 7 feet) 7 feet deeper.
I'd submit that everyone is narced below depths of 50 feet, with narcosis increasing with depth, of course. Those that say they aren't narced usually haven't dove with helium. Almost everyone surfaces from their first helium dive just astonished at how much they were missing, even at relatively shallow depths.
All the best, James