This debate is going in since 2004 and still nobody seems to know if the hidden _viewstate field is effects indexing. I believe for 100% that it is not issue at all. If the _viewstate is very large then you should probably have to worry about other things then SEO.
By now the search engines would probably ignore the _viewstate field and if not all hidden fields inside a page otherwise you would not find any .net page when using Google and not find any with high page ranking right?
Now type in nop commerce into Google. what do you see? yes all nice and indexed Nop Commerce top ranking #1 position. Hey guess what! it includes viewstate in the top of every page.
But if you really insist there's a lot of complex solutions online on how to disable Viewstate for crawlers, but here's a nice simple one. Simply add this code to your master page page load, so it runs for all pages:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not Page.IsPostBack Then
If Request.UserAgent.ToLower.Contains("googlebot") OrElse Request.Browser.Crawler Then Page.EnableViewState = False
End If
End Sub