Rim width has great influence on the shape and the performance of a MTB tire. A narrow rim decrease the weight of a wheel, wider rims provide more stability up to a certain point.
Every tire dimension is designed on a specific rim width. As a general rule of thumb, for every 1mm that a rim differs from the design standard (i.e. ideal rim width for a given tire), the tire will become about 0.4mm wider or narrower.
See the chart below for the recommended minimum and maximum inner rim widths for each MTB tire width. If you choose to run a combination outside of the recommendation, you may encounter negative effects on the shape and function of the tire.
Too wide: The tire's shape will be squared, possibly resulting in reduced comfort and cornering control, and higher exposure to sidewall cuts.
Too narrow: Less traction and cornering stability, tire may be prone to folding during cornering.