Why is this manga so weird? It has such a gloomy feeling and a weird vibe to it. Their reactions to stuff and dialogues are so stiff and unnatural. Every character acts like plastic.
Gloomy feeling.. ---> drama tag. Note the distinct lack of a comedy tag..
As for the "plastic fantastic".. Part of it is that the translation is
good and properly confers all the forms of japanese propriety.
And quite a lot of the situations we see the characters in are "formal" or business-like. With clear and distinct differences in status.
Given that japanese are
much more formal than "us here", in general conversations in those situations do come off as "stiff" or "plastic".
Because they are. Propriety trumps everything.
Only the Demon Lord acts anything remotely like we would, and
she is cast like a bratty insensitive barbarian who can do so because she is the damn Boss of Bosses.
All the comedy-tagged shounen try and do away with that formality as fast as possible ( all the "oh no..call me by -this" panels..) or by exaggerating the "not us, so they are doing things differently" things ( beastgirls doing hug-bombs and that kind of stuff ).
But that's part of the comedy, because it wouldn't happen in Real Life™ in Japan.
In Real Life™ in a "work" situation? Barbies pulled by strings following the deeply rutted roads of custom and propriety.
Until alcohol gets involved... And even then they've managed to slip in a ton of Rules..™
It's one of the things Japanese the more critical seinen manga poke fun at, or even ..criticise..