Look, 27 Dresses is fine. It's not offensive, it's not harmful, it's just... there. Katherine Heigl is likable, the premise has potential, but the execution is so by-the-numbers that even in 2008 critics were unimpressed (40% on Rotten Tomatoes tells you everything).
The message about standing up for yourself is solid, and the PG-13 content is appropriate for teens. But this is peak mid-2000s rom-com blandness—predictable plot, shallow characters, and a worldview where landing a man is presented as life's pinnacle achievement.
If your teen loves the genre or wants comfort-food viewing, it won't hurt them. But there are dozens of better rom-coms (10 Things I Hate About You, Crazy Rich Asians, even The Proposal) that deliver the same warm fuzzies with more wit, heart, or cultural relevance. This one's a C+ at best—watchable but instantly forgettable.