(Sigh) flag if you want, but this one had me in a couple hours, on the Pokemon sun and moon, episode 73, is Acerola's Mimikyu the only Mimikyu that can float?
"This shiny Mimikyu, known as "Mimi-Tan, is Acerola's only known Pokemon. Being the ghost of a deceased Mimikyu, It is able to float and pass through walls." - Bulbapeadia
From this, we know that Mimi-Tan can float and pass through walls because it is dead. We haven't met another dead Mimikyu is that anime, and I kind of doubt we will. So to answer you question, Yes (as far as we know), Mimi-Tan is the only Mimikyu that can float as of now.