Fire Emblem getting another character was essentially a band-aid that needed to be ripped off. Whether people like it or not, Three Houses was always a front-running candidate to get another character either now, or for Season 2. While I mostly expected the 5th pick being 3rd party, I always viewed Three Houses and Astral Chain as exceptions.
Frankly, I think the Fire Emblem hate, while understandable, ignores the context of the series and its relationship with Smash in the first place. We get Marth because he's popular and is a different type of sword-user to Link. As it's a new franchise to Smash, Roy is also brought in because of his new game coming in Japan, but also because he can be different from Marth. Roy was later brought back due to fan request during Smash Wii U DLC, being differentiated from Marth in the process, now playing nearly nothing like him.
Ike was added as a character from a newer game, which came from the FIre Emblem series, and Sakurai was looking for a heavy sword user. Despite Ike being quite fast in his base games, this could work for Smash. Ganondorf was not an option to declone, because there are abilities Ike brings to the table that Ganondorf does not, and he's a newer character at this point. Made sense to go with that decision.
Robin is added to Wii U because, unlike Chrom at the time, it made more sense to put in resources for a character that brings something quite new to the table. Lucina is added to pad the roster, just like Dr. Mario and Dark Pit. These three were selected because of resources that could be passed onto them, as they essentially share the shape, size, and moveset of their counterparts. This is not the same thing as, say, Dixie Kong over Diddy Kong, or some other choice. They were deliberate, but it also should be mentioned that Lucina was a popular character, and still is.
Corrin, while undeniably the more controversial pick, was added due to the search of a character from a newer game. The best justification I can come up with for this pick is that, frankly, with a mascot fighting game, main protagonists will always have the appeal over side characters to a general audience...the people you're targeting with most picks in DLC additions. I may be wrong, but there weren't really that many other recognizable games at the time that would have these requirements. Corrin also brought something new to the table.
Chrom gets in because he's a popular pick that requires few resources at the time, thereby getting into the game in the same way that Dark Samus or Richter Belmont get into the game. He is still functionally different from any other Fire Emblem character in the roster. Little time and resources, well thought out pay off in the end.
Byleth, our latest addition, brings a variety of weapons that people have been begging for in addition to their sword. They also come from a game that was quite successful (though they were chosen prior to the release of said game), but Byleth does bring something new to the table. Saying "they bring nothing" is a blatant falsehood. Other major releases at that point, such as anything Mario, Zelda, or otherwise do not have the same character pool of main protagonists that Fire Emblem has. We also have no idea how Game Freak works with Sword & Shield, while I don't know about them being against a SwSh fighter, I'm not sure if that would have been a production for the development team.
It is not Sakurai's fault that the Fire Emblem characters have similar designs- that is on Intelligent Systems. However, there's so much more to the characters than the color of their hair...that's something that I wouldn't think would need to be said at this point. You don't have to like Fire Emblem as a series, and you don't have to be alright with it after me suggesting these things, but I hope this provides some perspective. It's not all favoritism. I would argue it's not that cut and dry. At the very least, we're basically in the clear for any further characters until a future Smash game which...who knows when that will happen?