I’ve been searching for a Musharna holding a dream mist for about 20 minutes now. I put a fainted Covet Eyes Butterfree in the first slot of my party, but I haven’t managed to come across a dream mist Musharna. Considering it should be a 1/5 chance, is my luck just awful or does it not work?
There are easier ways to get Dream Mist. On digging further in this topic, there is a 5% chance of it being held by Musharna or Munna in the wild. According to the Bulbapedia Compound Eyes wiki page, Compound Eyes increases the chance by 50% of the current rate. 50% of 5% is 2.5%. That means that your rate of getting Dream Mist held by a Musharna/Munna in the wild with Compound Eyes is at 7.5%, so I suppose you’ll have to get lucky on this one or use other options to find Dream Mist. 
I’m unsure if you meant Covet or Compound Eyes, so I just went with Compound Eyes.
Yeah I meant Compound Eyes. Also, that Bulbapedia page literally says it’s raised to a 20% chance