Should fantasy distractors be avoided in multiple choice questions
I would say no, there's no reason not to, and in fact it might be better, based on the following rather speculative reasoning.

In general, despite the general benefits of testing for learning, multiple choice testing can also lead to subsequent memory for incorrect responses, i.e. it can cause students to learn false responses.


  Marsh, Roediger, Bjork, &amp; Bjork (2007) "The memorial consequences of multiple-choice 
  testing." Psychonomic Bulletin &amp; Review, 14(2) 194-199


This doesn't explicitly compare fantasy answers to other misleading answers, but intuitively, it seems that such "false learning" would be MORE likely to be reinforced in the future for plausible responses than for fantasy responses.  The reason is that fantasy responses are unlikely to ever be encountered again, while plausible responses might be, leading to reinforcement of the false learning.

See also here regarding avoidance of false learning from multiple choice testing:


  Little and Bjork (2012) "The persisting benefits of using multiple-choice tests as
  learning." Proceedings of the 34th Annual Conference of the Cognitive Science Society.