Regarding mountains, see
Mount Wycheproof.
Allegedly the smallest official mountain in the world at slightly less than 50m above the surrounding area. Albeit named so by mistake[1] rather than by obeying convention.
It might be hard to actually twiddle such a low mountain, but given that Z-levels are of indeterminate real-life height, normalising the actual height-map so that the largest elevation you
do have is made mountainous for game purposes might be a way to deal with this.
I live not that far away from an area where anything more than 3ft above sea-level is considered a hill, by local terms, and yet am currently in a city that prides itself on being built on "seven hills"[2] and has car-parks that have more variance of elevation in them than large swathes of the aforementioned region.
[1] Or possibly hubris or irony, on behalf of the surveyor, opinions on that vary, although as an official mountain is at least 500m high in some circles[3], it could easily have been a misplaced decimal in calculations and failed sanity check on the data, just prior to the classifying.
[2] Echoing Rome's claim. Although that sounds more poetic than empirical (or, indeed, Imperial), and nobody quite knows which seven they are, how big a lump is actually a hill, whether to count hills that are still rising on the way out of the city, where the actual city limit should actually be placed, etc, etc... YGTI.
[3] In the US, they have 300m, IIRC, and yet in the UK it is often said that there are no mountains in England (highest point, 978m ASL), only Wales (tallest being Snowdon at 1,085m, ditto). Then again it's "height above base", not height above sea-level, that is the standard (and not been bothered enough to Google those details). And there's also often slope angle requirements for different elevations as well. Steeper slopes allowing lower total elevations. Shallower slopes needing higher ones.