Gonna write out a big fat post about the concept of the Metagame.
If Marth is great and comes together because of the extra bonus of tippered hits, then he'll have results soon enough and Lucina won't, as she lacks them. If her design requires Tippers to make her risk/reward balance out, then she'll be unsuccessful as the metagame develops (aka as player knowledge and matchup-based playstyle adjustments mature).
But if Marth DOESN'T end up having tournament results, then we cannot argue that Lucina is leagues worse than Marth for not having a mechanic that doesn't help Marth any. She does have her own unique subtle differences, and those could potentially be worth more in the metagame. They aren't worth more on paper, but the metagame doesn't care what's on paper - it cares how the game is played out over years and years of refined play.
The metagame is a thing.
When players fight certain characters, they play differently. They know what their opponent's character is capable of, and adjust their play accordingly. Certain options of their may be less effective, while others become more effective. This changes and a flux occurs throughout a match and set. Sometimes there is also a player-knowledge aspect to the metagame - some players may know how to fight other players based on rivalries or past experience. But of course, the meat of any multi-character game's metagame is in fact matchups and adjusting to matchups.
When diddy can toss a banana at you or monkey flip, you space differently and play differently as a whole. Certain characters can combat his options, as can certain player playstyles with certain characters. Diddy players react to their opponent's reactions-to-their-diddy. Their opponent adjusts accordingly. A metagame forms. When you face a Captain Falcon, you adjust differently. Certain characters can adjust to other characters better than others.
There are so many variables to what makes a matchup in a character's favour, and it changes as the metagame grows. We know how to face off against Diddy because he is popular and player at a high level by many players, and those Diddy players have to adjust as a result to not be too predictable. And as they do, we continue adjusting to their adjustments.
The metagame is a complex thing, but definitely real. Every competitive game has a metagame.
Marth has been able to do well in melee and brawl as their metagames developed because denying him his ability to space and hit tippers hasn't been realistic. He couldn't be denied the utilization of his prime mechanic, and he was able to succeed. He was designed as a solid character who can adjust to his opponent's adjustments.
One of these reasons is because he was fast - when a character is speedy (either attack speed/endlag, or movement speed), you allow a large window of your metagame-based adjustments to be based on your dexterity as a player. You may not be able to adjust to a fast & deadly character as well if you played a heavy, compared to if you played a speedster. This is why slow characters fall by the wayside as metagames develop - you're prey to your frame numbers and can't adjust to the metagame as well (with convincing mixups, feigns, baits, and such).
Mixups on their own are a product of the metagame. You can't mix up if your opponent didn't expect something specific in the first place. Performing a mixup means knowing what your opponent expects from you (ie dthrow uair + uair as Diddy) and performing something else that they may not be defending themselves against. A metagame develops, and the dthrow metagame with diddy will continue developing. How it develops, we can't foresee. It'll just 'happen', and some characters will be better at future Diddy dthrow situations than others, which will be one of the MANY elements that affect the matchup.
Tippers are like any other mechanic - it can remain meaningful as the metagame develops (shine in melee) or become predictable and barely reap results (PKT2 in melee/brawl). Some mechanics are harder to play around and counteract than others. Marth used to be the premier ranged disjoint character, but this is no longer the case. We're left wondering if the Tipper mechanic will save him and be as resilient a mechanic as it was in melee and brawl.