Alvin and the Chipmunks are a fun escapist francise for all, I was entertained as a kid and am still entertained by them and many other "kids" franchises... movies and cartoons. To say that you can't receive joy from something targeted as for a younger demographic is just ignorant. Enjoy what you like.
My family went to see the last Chipmunks movie at the theater, not because the kids wanted to see it, but because my husband wanted to. We grew up watching the Chipmunks, and to us they are favorite childhood characters. I imagine this to be true for many people of my generation and older.
If you enjoyed Alvin and the Chipmunks as a kid, you can enjoy it again as an adult. It is also important for adults to watch movies with their kids, or even watch them separately if they must, as it gives them something to talk about and bond over. There is nothing wrong with adults giggling like children once in a while.
I think its a funny movie and comedy can be for kids and adults too. I personally like a lot of animated movies and the chipmunks in this movie are in that category. Its nice to watch a light fun movie where you don't have to think too much about the values and morals and just go with the fun. Sure adults can use a stress buster like this.