Depends, context can make or break a scene and that's why text based games work. But ugly graphics can utterly break immersion (For example the trap quest game has decent written scenes and etc but some of the faces the character model does ruins the vibes like an overly masculine and comical face when the character gets makeup). It's all about balance or commitment, if you're going pure text you need good plot, if you're going pure art you better have good art, if neither then you need to balance both so that the sum is greater than each part or else people will just skip to the gallery or straight up ignore it
Plot by far. I grew up playing NES games. Graphics are pretty unimportant to me in general, as long as there is a strong story there for me to witness.
Older graphics can be fine if the set up of the scene is done well through story. But if we're talking strictly visual novels, graphics are the main thing.
Most of the time it's game play, then it's plot and last performance.
Graphics aren't that important to me, i can play any graphics type game from 8 bit to full realistic graphics.
Graphics for hooking people in.
Plot for keeping people in.
I think that for this niche graphics simply matter more, the bar is higher than normal. If a game's eye candy is unappealing or too niche, then a large chunk of people won't ever even bother checking it out. That's how we end up with shallow games out there that do fairly well.
Meanwhile the plot doesn't have to mean "deep" or complex, just coherent and/or believable enough with decent pacing to get a pass.