I am not of indigenous Hawaiian descent, but I feel like I can answer this since I grew up there. FYI you’re gonna get a lot of different answers depending on who you ask, when/where they lived there, etc.

While I don’t find the “mainland”'s obsession with grass skirts and coconut bras particularly “offensive,” I do think people need to know Native Hawaiian women didn’t wear tops at all until Protestant missionaries arrived in the 1900s. There are no records of its official origin, but the coconut bra was eventually embraced by natives to an extent and used as an exotic marketing element for tourism. Now it’s permanently associated with the islands. Technically you could say coconut bras are traditional now, but they sure weren’t traditional before the islands were colonized.

But to answer your question, I personally do not find your typical “Hawaiian day” offensive. A little embarrassing and cringe, to be honest, but not offensive. I do think it’s disappointing, though, how most people accept things at face value and aren’t interested in learning about other cultures if it doesn’t fit the image they have in their head. Because there’s so much more to learn, man. I don’t think you’re “overreacting,” and I think it’s more than a little disingenuous (this next part is not aimed at you btw OP) to invalidate others’ concerns about cultural appropriation and write them off completely simply because the subject makes you uncomfortable. It’s a real thing, and it’s a discussion worth having. Who knows, you might learn something.

On that note, a fun fact: did you know that the iconic flower-print button-down short-sleeve collared shirts are not called “Hawaiian shirts” in Hawai’i? Yeah, only tourists call them that. They’re “Aloha shirts,” brah!