I think I'd choose to write something much ruder than that if I were compelled to wear one.

You can get them from Etsy - I nearly did but postage was around £11!

Tempted by one with 'PLACEBO' printed on it (just in case I run into the Vicar) - some are much ruder.

Regardless of what it says on the mask. There is plenty of evidence that they don't work against covid or any other virus, there is plenty of evidence that they are harmful to my health... I would have to be shot dead just to start thinking to put it on ! If I want to laugh on someone wearing it I'll do it; if I want to mock someone who is wearing it, I'll do it, but if I want someone to wake up or realize that this is a mistake, I'll speak to them. It is bit controversial for me to put the mask on to say something. Call me dinosaur, call me old or crazy, I don't care, but nothing works better than an old good conversation face to face with eye to eye contact.
Best example of this are millenials. Sitting 5 at the table and instead of having a good conversation and laugh, they texting each other, posting on social medias and similar... this world is upside down.