What's the name of the group that performs the song "Down Home Girl"
in the Honda Ridgeline truck commercial, featuring the siluohette of
the girl on the mudflap coming off and joining Yosemite Sam in the
front cab. I believe that the Rolling Stones originally sang this
song, but it sounds like a different artist in the ad.

The version of "Down Home Girl" in the Honda Ridgeline commercial is
by Nic Armstrong & the Thieves.

You can watch the commercial here:

New West Records: Nic Armstrong
http://www.newwestrecords.com/nicarmstrong/

If you'd like to purchase a CD containing the song, it's on "The
Greatest White Liar" (a mighty fine album which I recommend):

Amazon: The Greatest White Liar / Nic Armstrong
http://www.amazon.com/gp/product/B0007LXP8G

My Google search strategy:

Google Web Search: "nic armstrong" "honda ridgeline"
://www.google.com/search?hl=en&q=%22nic+armstrong%22+%22honda+ridgeline%22

I hope this helps! If anything is unclear or incomplete, please
request clarification; I'll be glad to offer further assistance before
you rate my answer.

Best regards,
pinkfreud

Excellent answer.  Speed of light response!

Thanks for the five stars and the nice tip!

~pinkfreud