It's certainly popular, from what I've seen. Through my job I have the opportunity to occasionally help with some events at my local library and we have a very healthy teen section, and a healthy population of teen readers who frequent the place....often for more than just books. (Our library has a lot of "teen only" events, such as gaming nights and an anime night, among other things.) To put it in perspective, our teen section is large enough to have not only it's one section of shelves that sits among the adult books, but also an additional separate room/wing thats nothing but YA books. In fact, I'd argue the teen section is the second largest section of the library, with the first being the section aimed at little kids/elementary schoolers. ( and even then, thats mostly due to the fact it includes not only books, but computers, educational toys/activities and a lot of other things while the YA section is basically all books, sans like two shelves of manga that get lumped in with it because we don't have an actual comic/graphic novel/manga section.)
That being said, is it as "trendy" and "big" as it was before? Not by a long shot...as others have stated, there used to be this sort of mentality that YA was the big ticket, hot selling catigory, and there was a sort of constant push for best sellers. Now, YA is not really the "big ticket" it was before, but is certainly not unpopular....far from it. I'd actually say thats whats happened with YA is that it's transitioned from a fad to an actual, legitimate genre with it's own devoted followers and community. The whole emphsis on fads and "big ticket" books has been pushed to the wayside in favor of just putting out good stories, reguardless of whether they follow any kind of market fad....
In fact, one could say that the de-fad-i-fying of YA has made it more popular rather than less so, since there is actually a lot of innovation going on in the genre that a lot of writers are not aware is a thing because it's happening in places they aren't looking. In particular, the much sought-after "boy book" that so many have said does not exist because "teen boys don't like reading" is actually growing in popularity rapidly on sites like Amazon, but such books don't actively market themselves as YA. You'd be surprised, but there are a few communities of largely teen boys and boys in their early 20s who obsessively devour web-novels and self-published novels that are actual novels...not comic books...not graphic novels...actual, prose books with no pictures beyond the cover.
Places like Royal Road Legends exist and are innovating the genre by making the "boy books" the publishers refuse to touch and it's great because it shows that the genre can grow and innovate itself even if publishers may not be willing to change with the time. While a lot of these books advertise themselves as genres other than YA, they have all the elements of a YA story in most cases; protagonists are often teens, the coming of age elements and other trappings of YA are all present....except those YA elements are often intermixed with very over the top, anime and/or videogame-inspired action and powers, a distinctly male voice/male protagonist and in some cases actual, videogame style stats and number crunching being important elements of the narrative. (These stories also tend to be written in 3rd person, while most mainstream YA goes for first person.)
Granted, there is a huge issue regarding sexism and the portrayal of females in a lot of these stories (Many of the boys who read in these genres will flat out refuse to read stories written in the same style/genre that have a female protagonist (In fact, there are quite a few female writers in these genres...its just most of them end up writing male protagonists as a result of this.). Also, the tired anime trope of the male protagonist with a "harem" of girls who have little character beyond being prizes for the protagonist to win/damsels for them to save comes up a ton in these genres.). However, thats a whole different can of worms that I think will largely be worked out as things evolve; already there is a sort of "anti-harem" sentiment starting to spring up among some in these communities and as stated, it seems like more girls are starting to both read and write in these subgenres... so only time will tell what comes of that...
With all that out there, I think we can safely say YA is a thing, and it seems like now rather than being "the" thing it has evolved into being a thing that will be here for years to come... a thing that grows and evolves instead of one that simply exists to push out trendy books. In all, I'd say this is the best era to be a YA writer we've had so far...so embrace that.