I'm sure plenty of people found Team America offensive. Personally, I find most of it funny (there are definitely parts that are stupid/tactless/heavy- handed, but that's Matt Stone & Trey Parker productions in general).
Listening to the theme from Team America on 9/11 is not for me, but I don't find it blatantly offensive either. Nothing wrong with poking fun at America and/or enjoying a bombastic/comedic song, but I'm betting 99% of the people questioning your decision are just questioning your timing.
9/11 is a touchy subject, people died, and America experienced a new sense of vulnerability and instability that we hadn't since probably Pearl Harbor. That's big stuff, and people handle that with differing emotions based on their experiences and relationship to the day itself. Especially those who lost friends and loved ones.
Memorials/commemoratio ns/rembrances are tough things culturally to manage, and 9/11 is full of a lot of cultural and political baggage. I'd just be careful how you approach the topic.
As an aside, I will occasionally watch 11'09"11
on or around September 11th. Not every year, but some. It's a collection of short films from around the world that explore 9/11 from multiple different perspectives and approaches. Some are funny, some hard to watch, some really poignant, but all really interesting ways of seeing that moment from a view that is not the one most of us are used to.