She's not a main character so it's weird she was even added to Smash. She's playable in the second half and it doesn't switch to her perspective like Lyn to Eliwood or Sigurd to Seliph. You never get to play the game in her perspective and it's not like Alm and Celica where you switch or Hector where you unlock her perspective.
She is not a main character and yet she was added to the game because she is girl Marth so she was really easy to implement development wise
Lucina is the 3rd most popular character in the entire franchise, that's why she was added.
Who among Roy, Marth, and Ike is she more popular than? ---