Yeah, it's easy to dismiss it as tin foil until you start to look at the details. Look at cities that waste money on things like unused bike paths and railways that go to nowhere. Lots of micromanaging about how much water you can use or exactly how every structure in your yard should be built. A lot of it is impractical ideology that lacks common sense.
There are states that have considered legislation limiting or even banning some of Agenda 21's provisions.
And notice that you really won't hear the term "Agenda 21" at a local level. Guess the advocates finally realized that using words like "agenda" really does sound conspiratorial!