Hi hi i normally have ideas and wanna make horror themed content usually based around mental illness and sensitive topics. I don't plan on showing anything graphic but the overall theme would be a bit uncomfortable. The last game i made didn't have any explicit content except the ending game card describing what the player found. So i'm just curious as to what would really fall into the nsfw for a horror game

I don't think so, as NSFW only concerns graphical content and not the theme itself. So you should be fine. But don't take my word on that, better wait for a professional to answer this.