The biggest hit is it "wastes" another Mario spot, but I don't really even think they care about that and will just add as many Mario characters as they see fit; that certainly didn't stop them with FE.
Except not really. There was no Mario slot. It was a "surprise character" only. It would've been at best some other Mook that had a fun moveset Sakurai could think of.
Every single Fire Emblem character except Marth were circumstance-based.
Ike/Corrin/Byleth/Robin: New games already out or coming out within the right period of time.
Roy: Special advertisement due to being in the clone slot. Sakurai was convinced go with him. Corrin and Byleth also fit these.
Lucina: Was a costume already modified to be nearly Marth's height way beforehand. This is the only reason she was an Echo.
Chrom: Super easy Echo and was a huge popular request, especially after the situation with him not being in Smash 4 and then getting done dirty by both the Lucina/Robin reveal and the Palutena's Guidance. He was already popular as a choice(as he's one of the two core protagonists of Awakening, the other being Robin/The Avatar. Whichever name you prefer), and people expected him. Chrom was also planned at some point but scrapped for Robin due to him not being unique enough between Ike and Marth.
They're only being added due to unique circumstances. Frankly, Rosalina is a lot like the unique newcomers in FE. The right character at the right time. Fire Emblem also doesn't have recurring characters, so they can't "stay for a while to earn a spot" like many others. It's akin to how Pokemon is since effectively Lucario, but moreso Greninja and Incineroar.
PP is nothing more than a surprise Mook character that Sakurai had a quick idea for to make a fun DLC choice. Daisy is a popular request and an easy Echo. Don't forget that the Mario series has a lot of unique newcomers and some clones/echoes too. Let's note them too;
Unique: Mario, Bowser, Peach, Rosalina, Bowser Jr., Piranha Plant
Clones/Echo: Luigi, Dr. Mario, Daisy
Unique: Marth, Ike, Robin, Corrin, Byleth
Clone/Echoes: Roy, Lucina, Chrom
Luigi and Dr. Mario literally were added due to the fact they had extra time. Luigi would've obviously been in Melee otherwise, but keep in mind he wasn't actually planned for Smash 64. Same with Dr. Mario. Daisy was part of the base game plan, as was all the FE characters in Ultimate bar Byleth(who is rumored to have been thought of for base, but the game was delayed anyway, so that didn't work out. This suggests that Nintendo only suggested Byleth cause Sakurai already had the idea, but he still noted it wasn't his "personal pick" either). PP was added later on as a DLC idea. The only bonus DLC ever thought of was Mewtwo otherwise, who was used in a similar way, to get people to buy the game early to get it for free. That's all they have in common.
Don't get me wrong; PP wasn't the Mario character I wanted either. But it's a surprise addition that only took a slot away from an actual surprise addition at best. And if you look closer, it'd have likely been a mook or an unimportant villain type character. Sakurai's own words specified that it was both iconic and not "a hero" either. That doesn't leave a lot of options for what it could've been if it wasn't PP. So what else would've made sense in that context? Another mook? Sure. A ton of them are very iconic. Another non-heroic Pokemon could count too(Meowth for instance). Mewtwo is kind of similar to that, if only cause it's presented with an evil laugh in Melee, but that's much more stretchy of "villain" than an evil mook.