i think that they are mythical cause they are events but what do you think?
Regardless of how rare Rotom is, none of Rotom's forms or Rotom are mythical or legendary, just rare.
Rotom is not mythical. All of its forms can breed. The forms were event exclusive for only one year, because the form changing appliances are accessible in HGSS.
Also Bulbapedia's mythical Pokemon article doesn't mention Rotom.