Eh... "Mood" and "atmosphere" aren't whipped up out of nowhere though. The film's content which would include onscreen violence in the story or indeed visuals. They contribute to mood and atmosphere.
Gremlins one is a film bursting to the seams with horror and I would not argue one bit with someone saying it's obviously a horror/monster film. The content and visuals certainly make that case. Yes, the gremlins have many moments where they are used for levity and could be viewed merely as anarchic, even charmingly so.
But take a closer view and you get that in that first film the moments of whimsy and humor are far outweighed by the scenes and moments where they are presented as gleeful murder machines. And it gets pretty rough at times. The scenes with Billy's mom getting attacked and fighting them off is intense. Billy gets attacked in myriad ways by the creatures including getting clawed up, taking crossbow bolts, chainsaws, and guns. And none of that is shown in a funny way. The violence is presented as mostly threatening to the audience and outside of the Snow White theater scene and the bar scene, they aren't too kooky. They are the obviously near demonic forces that have to be stopped. Interesting side note is that Gremlins was along with Temple Of Doom and some other popular movies, a main reason to create the PG-13 rating.
Compare Gremlins and its sequel and you see what a difference content and visuals make in creating mood and atmosphere. Unlike the first movie the creatures are a lot less, well, threatening. The violence and SFX are less palpable and way more cartoony. Two films, same director, same concept and characters but the mood and atmosphere between them is night and day.