mask bikini
A mouth mask worn on the breasts as a bra or a bikini top.
This is typically is shown in various ways, but generally in a creative ad hoc fashion involving tying, interlocking, or otherwise binding multiple masks together to be worn as a larger outfit (see Danbooru post #3830413), sometimes also incorporating mask panties. Ironically, the character involved may or may not additionally being wearing a mask properly over their face. This theme developed in response to the coronavirus pandemic of 2020 where mask usage was brought into stark focus.
For the converse of this tag, where a bra or bikini top is used as a mask, see bra on head or bikini top on head. For bikini tops that aren't actually made of mouth masks, but which adopt the specific strapless design associated with mask bikinis, see eyepatch bikini.