I'm not sure about the affect your talking about, but I do know that it is very unlikely that StumbleUpon traffic will click an Adsense ad. If your hub has been stumbled it means someone felt it was worth sharing which is a very good thing.
StumbleUpon traffic comes rapidly and is very ineffective if your looking for ad clicks. This means that your adsense stats could be affected in a negative way, but not so much that you should worried about it.
If someone has submitted you to StumbleUpon you should be happy, it means they thought your hub was worth something.