"too diverse" is really a very strange term. I don't know where those people live who say that, but especially in huge cities you can find enourmous diversity of different ethinicities and races. So there's nothing unrealistic about it. Especially since your comic takes place in Paris people can't expect for your whole cast to have for example the same skin colour.
Maybe that would make sense if your story would play in, let's say, a small rural town in South Germany where everyone knows each other for 50 years and barely new people find their way (been there, and hell no).
Only spoken from the point of characterdesign, it's pretty boring to me when all characters are of the same make up. Characters in mangas for example often suffer from the so called same-face syndrome, where they just gotten put on a different wig and eyecolour. It's.... boring. I simply enjoy characters who are not only different in character but also physically. Makes it easier to tell them apart haha.
About sexual orientations: if you want to make your cast gay/ace/bi, whatev, make them all gay/ace/bi whatev. There can't be enough representation. Orientations aside from heterosexuality should be normalized - because unfortunatley they still aren't - and comics are a great way to take a part in that process. Second, it's your damn comic. Do what you think is right and suits your story/characters best.
I certainly agree with the argument that you shouldn't make characters just for the sake of diversity and leave their personalities flat as a pita bread. I've read comics where it felt like the creator was trying to meet some sort of diversity quota in regards to mental illnesses, which just felt off during reading. Maybe my problem was more that all of those things were revealed in quick succession, it was just thrown out in between dialogues withouth any proper context. It was probably done with the best intentions, but wasn't convincing to me at all.
Also, in my opinion, sometimes it's better to take a step back and save a certain character for a different story(line) to give them a proper voice, instead of just being one of many in one storyline. But that's just my personal taste, because I find it easier to focus on a couple of characters and their story, instead of a whole bunch.
In my own comics I prefer to portrait the things that are considerd "different" in the real world as normal in my comic. Be it a bunch of different ethnicites and races living in a small town South Germany-style together, or sexual orientations. For example, I have one character, a boy, who comes across as rather feminine for "worldly" standards, wearing a skirt and all. I won't talk about that subject within the story because it's not an issue there, and I also think it shouldn't be an issue anywhere. When my readers asked questions about him I couldn't pretend that in our society it wouldn't raise eye-brows. So I answer those questions, and people are satisfyed.
Ah, whoops. Damn, I hope I make sense haha.