Saying that "Las Vegas is Disneyland for Grown Ups" isn't a stereotype. It's a point of comparison. Everyone knows what Disneyland is. It's an American institution. It's fun. It's well themed. Now imagine a place that instead of rides there's gambling. Instead character shows there's nude cabarets. Instead of Coca-Cola there's Jack Daniels. That's all it is. It's not meant to be taken literal.
If I told someone that Florida's Space Mountain is like Disneyland's Matterhorn but in space, it doesn't mean there's an abominable snowman in their Space Mountain. It just means that there's aspects that are very similar. It's a point of reference.
Las Vegas isn't ALL bad. That's a stereotype some folks are being pretty immature and ignorant about as well.
I'm an adult and I LOVE Disneyland. But I can still say that Vegas is like a Disneyland for grown ups and not lose any sleep over it. If there are adults out there who think Disneyland is for kids, well... too bad for them! They're missing out! And if I want to correct them I will. But otherwise, no big deal.