It's piped on with royal icing.
Or do you suppose it is molded fondant?
could be cut with a cricut too
I've seen this done with fondant cut outs, that had piping on top of the cut outs
It's piping done with royal icing, as TexasRose suggested, or brush embroidery or a series of lace pieces made of fondant in a mold and attached to the cake. Brush embroidery is a technique in which the main lines of a design are piped on with RI and then they are drawn toward the center of the design with a paintbrush. Hard to say which without a close-up (I'm so blind!!! Another reason why my avatar is a bat !).