In-game, she shows up with her mask, and presents herself as Masked Marth, so the corresponding tag should be added. Extremely likely to see her along with Summer Mia, the main character of the Duo Unit she's part of - Summer Lucina isn't her own unit in-game.
Note that it's possible (especially for older artwork), to depict Lucina in a swimsuit without it being the one described above, in which case this tag should not be used.
