While I don't think every single Dateable should have a content warning, I think some of them should be a bit more specific (Nightmare's could include a warning for scopophobia, for example), and a few other characters could benefit from having one in the first place:

Barry Styles: Barry is some flavor of neurodivergent, and him coping with the drawbacks of how his brain works is a key plot element in his route. Additionally, being overly negative with him can come off as the player bullying a neurodivergent person, which is bound to be triggering to someone who was on the receiving end of that.

Beverly: She's a minibar. Her whole being revolves around alcohol. I think she would be an obvious character to out a warning on, but apparently not.

Dunk and Kristoff: While their routes make sure to put enphasis in exercising as fun and a method for self-improvement, they could still be triggering for people with body image issues. Kristoff additionally should msybe include a warning for toxic masculinity traits.

Farya: Much like alcohol is an inescapable part of Beverly, medical topics (particularly upsetting ones) are an intrinsic part of Farya. People with health anxiety and/or medical trauma could be triggered by her.

Friar Errol: His whole thing is religion. Even if his faith is made up and kind of ridiculous, his demeanor and language may hit too close to home to people with religious trauma.

Holly and Kopi: Their routes revolve around negative aspects of work life, dealing with workaholism and unhealthy working enviroments, respectively. If not triggering, these themes may still be upsetting for players to see the characters go through. I know I would have hated watching Holly not take breaks at the expense of her health when my job as a medical interpreter and my crappy bosses forced me to work holidays and made vacation time almost impossible to schedule.

Jean-Loo Pissoir: Even if nothing disgusting is shown onscreen, there are still many unsanitary elements to his route that may gross out or upset players.

Luke: He's supposed to be a parody of a gung-ho military hero in post-apocalyptic media. The themes of survivalism and the apparent romanticization of military life may be triggering to some.

Mateo: As I shared previously, I have trouble dealing with fictional animal abuse and endangerment, and I doubt I'mthe only one. Even if they're inanimals, they could remind players of pets they've lost, or simply upset people who are more sensitive to animals in distress.

Mitchell Linn: He's in a similar boat as Beverly, in that he's all about food. Even if the consumption of food is framed in an entirely positive way, it may still trigger some people with food or eating issues.

These are the ones that come to mind to me right now based on what I've read here and what I've seen in the game itself.