Do 27.5" wheels fit a 29" mtb?
They fit, in the sense that you can install them in the frame (if the axles are correct, hub width is correct, etc). The problems are:


The bike stands a few centimeters closer to ground. Which means less ground clearance for obstacles and cornering. Frames designed for smaller wheels take this into account
Less trail at front, which affects steering
Lower gearing because of smaller wheel radius
You still have the long rear fork of frame designed for 29" wheels and the associated weight distribution and clumsiness.