It doesn't only use biplanes, searching "wingwalking monoplane" provides these examples, of some low wing monoplane and high wing monoplane.
Whatever the aircraft, it has to be able to fly slowly.

[![enter image description here][1]][1]
([source][2]) (not 100% about this one but it looks like one Klemm aircraft)

[![enter image description here][3]][3]
([source][4])
(Here one quite rare danish SAI KZ III equipped with a hatch behind the wing, allowing acrobat Palle Johnson to get out.)

[![jaromir wagner][5]][5]
([source][6])
(Jaromir Wagner, Czech wingwalker holding speed record wingwalking at 270km/h here on a Britten-Norman Islander aircraft)


  [1]: https://i.sstatic.net/mhui3.jpg
  [2]: https://www.aerotime.aero/aviation-blog/wp-content/uploads/2017/06/http-2F2Fa.amz_.mshcdn.com2Fwp-content2Fuploads2F20152F092Fwingwalkers-1.jpg
  [3]: https://i.sstatic.net/mmIDj.jpg
  [4]: http://www.natureandtech.com/wp-content/uploads/2010/08/wing-walker-300x173.jpg
  [5]: https://i.sstatic.net/CdiAd.jpg
  [6]: https://fly.historicwings.com/wp-content/uploads/2012/09/HighFlight-JaromirWagner3.jpg