I don't think it could ever be "easily", since I see no way Lucina could ever not be top tier. She has incredible hitboxes, frame data, speed, kill power, edgeguarding, one of the best OOS options in the entire game, etc. Roy might also be top tier, but unless Lucina gets some kind of nerf I can't see anyone being significantly above her.
Speaking of, did Lucina gain more damage compared to Marth relative to Smash 4? In that game, it was usually considered to be the case that Marth players should play both characters and switch out for certain matchups, not the "Lucina is just straight better all the time" situation it is now. If so, reverting her back to her previous level of power seems sensible, Marth should feel significantly more powerful when he hits tippers. In general, I feel like Lucina should probably not have very good kill power at all to make up for how absurdly solid she is in every other way, she definitely has the tools to consistently win neutral and close out stocks that way.