As a boss. Yes. As far as mechanics and properties no. It's out of character. It's complicated.
Because he's a savage murdering monster who loves to cause death and pain. Him immediately becoming someone's slave just because they hit him is incredibly unfitting.
No, it isn't complicated; that's just reading too much into it. And Jesus, don't try to make it sound dramatic saying "become someone's slave". That is just a gameplay feature, is not intended to depict a character's trait or personality or the do do's and don't do's of what he/she would do in a situation in their home series.
Because if we start to look at it that way then we have too many other examples that would be an "out of character" moment and pretending that they conflict with them would get silly, like:
-Ganondorf and Link are mortal enemies yet you can have them fight as allies in a team match against Zelda. Ganondorf, the King of Evil, would never fight along the hero of Hyrule.
-When playing Smash Run as Bowser, Koopa Troopas and Magi Koopas will attack him., their own king! Same with Ganon and enemies like the Stalfos.
-Mario can beat up Peach. He is a hero, he is supposed to save her, not beat the crap out of her. Is not like him to do something like that.
-The Halberd's weaponry can target and attack Meta Knight. That ship is run by his own men, they should turn on his enemies, not him.
-Peppy, Falco and Slippy and the Great Fox may shoot Fox in the Corneria stage. They've always showed loyalty and would never back stab their own leader like that.
-And the biggest offenders: Assist Trophies. We were talking about how brutal Ridley can be and how he doesn't do someone's bidding, right? So I guess that unlike him, Mother Brain, Andross, Ghirahim, Dark Samus, Shadow, the Ghosts, Samurai Goroh, etc. are far more submissive in their home series then. It's a worse case than Ridley's since they don't even oppose you and immediately assist the character who called them. Fox is capable of summoning Andross; Samus can call Dark Samus; and Link can do the same with Ghirahim; those villains will HELP their mortal enemies without even resisting at all. Out of character and demeaning indeed, right? After all, a lot of them are big bads who would never "slave" to someone like that.
And yet everyone is ok with that. But since it's Ridley now people want to feel offended, because *sigh* it's Ridley and that subject somehow overrides logic and reason. It's hypocritical.
I think you can get my point here. Ridley's appearance and how he works is not a personal attack on his person, it's just an inoffensive gameplay feature. He is no different from a lot of this examples and out of all the NPC characters he has received the best treatment of all (second best thing besides being playable), he has appeared in all Smash games in some form, he was a story boss not once but twice in Brawl and now a stage boss in this iteration. And do not tell me he is a special case; people always make him an exception for some reason.
I think it's badass how he can fight alongside you, it does not mean that he is some lowly lapdog or something. It's not right to pretend that he is being screwed over somehow, this is the probably the knee jerk reaction from people who wanted him playable but that is a terrible way to cope with the loss. So don't tell me again that his current appearance is unfitting because that is not case. It's stupid to say to put that way and it's a childish reaction. Not cool.
However...
Because he's a savage murdering monster who loves to cause death and pain. Him immediately becoming someone's slave just because they hit him is incredibly unfitting.
This sentence is false. In Metroid Prime Corruption, Dark Samus enslaved the Space Pirates AND Ridley to a point where they pretty much became her cult followers. Even making Ridley undergo full corruption and leaving him guarding the Leviathan in the Pirate Homeworld on her orders. And you thought Ridley in Smash 4 is incredibly unfitting. Please, let's not lie to ourselves.
There are better ways to be funny than repeatedly making a character look like a total loser, and rubbing in the fact that you can't play as him. I've never even touched a Fire Emblem game, and yet I think it's a bit harsh.
Chrom is also descended from Marth. If I was Chrom I would not find such teasing humorous.
Like the Ridley situation, you are just being oversensitive. You might as well feel offended for Colonel Campbell "insulting" Luigi or Snake disliking Sonic. Let's not be like that and stop digging for any minute detail to get angry at. Get yourselves a sense of humor, gentlemen.