Is it possible to decrypt and view ViewState values?

Sure. ViewState is simply base64 encoded (unless you specify that it should be encrypted). Here's a link to someone that wrote a ViewState viewer. Here's another by Fritz Onion. You probably will not be able to directly modify the ViewState (i.e. outside of code) because ASP.NET has checks in place to specifically ensure that nothing has tampered with the ViewState. See the EnableViewStateMAC setting for more.

Update

Thanks to link rot, the links to the various viewers are no longer valid. However, a simple search for "viewstate viewer" can find others on the Internet such as ASP.NET ViewState viewer