First of all. in terms of political credibility:
"From 1989 to 1993 Peter Garrett served as president of Australia's peak environment organisation the Australian Conservation Foundation. Significant results for many threatened areas of the Australian environment including Coronation Hill in Kakadu, Shoalwater Bay in Queensland, the Queensland Wet Tropics rainforest and Jervis Bay in New South Wales were achieved during this time."
"In 1993 he joined the International Board of Greenpeace..."
"In 1998 he again accepted the presidency of the Australian Conservation Foundation, a role which has seen ACF grow strongly, develop partnerships with Non Government Organisations, progressive business groups, and expand its campaigning into marine conservation and northern Australia.
He received the Australian Humanitarian Foundation Award in the Environment category in 2000 and in 2003 the Order of Australia (Member General Division), for his contribution to environment and the music industry."
"Peter Garrett serves as adviser and patron to various cultural and community organisations including Jubilee Debt Relief and was a founding member of the Surfrider Foundation."
(For those unaware, the Surfrider Foundation is a non-profit organisation working to preserve oceans, waves & beaches globally).
While these positions may not all specifically be political in nature, I think it at least provides him with a significant background in caring for both our country, and our world. Personally I think it's refreshing to see someone whom it's safe to assume is not entering politics for personal gain.
I don't see where the basis for this criticism arises? Perhaps you'd prefer his position filled by some lazy, out of touch politician motivated by nothing but greed and a nice retirement package.
In terms of his background in entertainment, I don't see why that has to be a bad thing either. A significant portion of his work in this field was spent dealing with the same issues, so it's not really as huge a transition as it initially sounds. With any luck his profile will attract support to these causes and encourage more people to give thought to these sorts of issues (Other political parties included).
Funnily enough, more information relating to Peter Garrett's previous activity is available at
http://www.petergarrett.com.au/ , perhaps it'd be an idea to read (at least)
Hope that's not too much of a rant, I'm just a bit shocked that not everyone thinks that it's a champion idea