She has potential as a character, though I feel she's really limited as a character compared to Marth and will always be in his shadow in terms of viability by a fair amount. Previously, I felt Lucina was slightly better than Marth due to her disjoint being all around safer and her having more reliable low percent combos, but after I began playing both characters more, I feel Marth is definetly the superior of the 2.
While Lucina's damage racking game from about 0-30% is generally better than Marth's, past these percentages, Marth's ability to rack damage is overall superior just because he's either doing more damage from his tippers or because his sourspots launch the opponents in more unfavorable positions for tech chases / combos. Additionally, Marth's moves also have significantly greater KO power when tipped and between tech chase setups from his sourspots that Lucina lacks or just stray hits in neutral, he is able to land these tipper hits fairly easily at KO percents, making his KOing ability significantly better. Because of this, I feel Marth's moveset is significantly better past low percents just because he can pick and choose between using his tilts and aeirals to setup tech chases for damage or KO, while Lucina's moveset is, comparatively, significantly worse at doing both.
(Additionally, I find her dancing blade much harder to properly combo into itself. Not sure if this is due to the moves greater KB between hits of the move or due to the greater hitlag that the move has. This is likely just a personal issue, but I thought I should bring it up.)
Thats not to say Lucina doesn't have her advantages though, namely her higher safety against shields, making walling out some characters a bit easier and giving some of her moves like her F-Smash much greater utility. Additionally, she can make much greater use of the first hit of her Nair for combos / kills. However, I feel these small advantages aren't enough to make up for Lucina's noticeably lower reward when compared to Marth.
While I feel Lucina is noticably worse than Marth, I feel as a standalone characters, she's great. Like Marth and to a lesser extent Roy, I feel that her entire kit is good enough to overcome any MU. She still has many of the key advantages that Marth such a threat, namely his amazing options in neutral like SH and FH Fair, DB, Up B OOS, etc.
Main reason I'm making this post is because I see all too often people claiming that Lucina should be ranked alongside Marth due to being "the same character" or just "slightly worse" when this simply isn't true. The different knockback and damage make her entire moveset much worse in advantageous situation. This is also why I feel her results will also stay mediocre, as most players will likely stick to playing Marth in a competitive scenario.