It is a bad thing to have supersonic fan blade tips, just like supersonic propeller tips are best avoided. But in turbofans it is a price worth paying, because the faster tip velocity means higher dynamic pressure, and the pressure difference between both sides of the fan blade grows with the square of their velocity. This makes the high thrust levels of modern turbofans possible.

[![Propeller efficiency over speed][1]][1]

Propeller efficiency over speed (picture [source][2]). The plot for fan blades would look not much different. The very thin, uncambered airfoil of a supersonic propeller and the added wave drag lower the maximum efficiency, but hold efficiency up into supersonic air speeds.

Note that the propeller on the [XF-84H Thunderscreech][3] did move at supersonic speed. There is nothing inherent in propellers which prevents their tips from moving faster than the speed of sound. On the other hand, the big diameter of a prop requires proportionally more torque to keep the prop rotating against the drag from the supersonic tips. Thus, a fan engine requires less torque to reach supersonic tip speeds on the fan blades.

Also, the [shroud of a turbofan engine helps a lot][4] to make the noise from supersonic tips manageable. The XF-84H's noise made people literally sick.

Generally, a fully subsonic fan would be more efficient. But then the diameter would need to be as large as that of big turboprop engines, and the shroud would become impossibly heavy and produce too much drag. The high dynamic pressure on the fan blades is needed to produce the thrust with the relatively small diameter of a turbofan.


  [1]: https://i.sstatic.net/tpaZ1.png
  [2]: http://www.enginehistory.org/Allison/XF-84Propulsion.html
  [3]: http://aviation.stackexchange.com/questions/21683/did-the-xf-84h-thunderscreech-really-cause-900-sonic-booms-per-minute/21700?s=1%7C0.1966#21700
  [4]: http://aviation.stackexchange.com/questions/11884/do-nacelles-around-aircraft-engines-help-in-reducing-the-engine-sound/11891#11891