I only remember Green Day from when I was in college. My son liked them whenever the American Idiot album came out and they reached a whole new level of popularity, but he was also nine. So, I suppose that's my evidence to know they're marketing Smackdown as a kid's show.
Most adults probably don't like Green Day. By the time Green Day got popular, I was already talking about how my generation's music was better. Kids and teens are really the only people who listen to Green Day, other than people my age having a mid-life crisis.
Also, the fact that Smackdown comes on Friday nights is probably a good sign that most 18-30-year-olds aren't home to watch it, anyway. It's clearly meant to appeal to the teens and kids who will be home or at least in front of a TV on Friday night. I'd probably watch Smackdown a lot more often if it were on Tuesday, Wednesday, or Thursday.
So, no, even though I don't like the current Green Day song, I understand why they use it. I also don't like that Smackdown is such a PG show, but again, I don't think I'm meant to like it. It's an annoying song, it's a predictable, poorly written show, and I think that they could do much better, but I think that's what we have RAW for. RAW could be a lot better too, but I think Smackdown is more of a show meant to push action figures. That's why Randy Orton is the big babyface on that show.