Don't worry about your lack of experience in that area
You're saying that this is a YA book, so presumably, your characters are not really all that old. Which - on the one hand - means that your lack of experience and awkwardness might actually be a lot more fitting than you might imagine, but also that it is very unlikely that your book will become more grown up by adding a love triangle. Although it sort of depends on what you mean by 'grown up' exactly. The fact that the characters are not all that old and mature yet will probably (and definitely should) come through with any romantic relationships they might form, making their young age more obvious than it would have been without.
Then again, it is absoluteley ridiculous what behaviour people in YA books get away with while the fans think it's charming. Probably because the fans are pretty young themselves. So even if you 'screw up' here or there, if your book is decent enough, people will ignore it or even come up with excuses (just think of all the extremely old men coming on to and actively stalking high school girls in vampire novels).
Should you add it? Absolutely!
Not because love triangles are fun (in fact, they tend to be very tedious to me). Not because they make the book more grown up (they won't).
But simply because the author (that's you) said:
I feel like it would add another subplot to my story and make it more
As far as I'm concerned, if you as the author think something would make your story more interesting, there's really no question about it. You don't need any additional reasons. Add it.
Then again, if what you meant by that sentence was that the simple act of adding any subplot would make your book more interesting, no matter what it's about... you might want to improve your main plot.