Eh, art is hard. Don't let those modern artists fool you, it takes a lot of work to be good at it. I'm sure some of them know they aren't the best, but share as a means to spread their love and contribute. You're, by all means, taking the best approach by just letting them do their own thing, tbh. There's no reason to lie to them if you genuinely don't see anything worth complementing, but just as a hazard, you also shouldn't assume anyone is lying if they do complement the art or even just certain aspects of it.
Not accusing you of anything, of course, I'm just someone who is only good at one specific thing, and is very jealous of very simple or even actively terrible art that can capture something or make a form I still struggle with.