Mountain bikes used to have fairly short reach measurements (the distance between the middle of the stem cap and an imaginary vertical line taken from the center of the bottom bracket) and long stems. But as reaches have got longer to improve bike stability, stems have got shorter to improve handling – alongside longer bars.
Most stems tend to come between 30 and 50mm these days. Shorter stems aren't technically "better", they will give your bike sharper and faster reacting handling.