Those old mtbs definitely make great commuter bikes. My wife and I both have awesome townie-errand-commuter bikes built on rigid MTB frames from the 90s.
IMO these work better for this purpose than newer MTBs.
However....
1- there are very good purpose built commuter bikes available now.. The offerings and options for utility bikes has gotten really good in the past 10 years. At least as good as the old MTB frames.
2- As far as actual mountain biking...... those old mtbs don’t even come close to newer designs. Heck, even designs from the 15 years ago were way better.. MTB design has come a looooong way since the early 90s.