There's this whole symbolism in California with the show. In the 60's New York was decaying and there was this total optimism with people about going out to California, the new American dream. Weiner has loved driving that point home, that the Mad Men are stuck in the past and lying in their own filth that now with the end of the 1960's is visible to even themselves. But they go out west and suddenly they have a totally new perspective on life and everything feels just right.
The Mets pennant I always thought signaled this desperate clinging to the old ideals of the New York, east coast American dream. Born out of the ashes of the Giants and Dodgers bolting for the Pacific.
It makes sense in a historical context and I've really liked it in the show despite California seeming almost dreamlike.
I think it's just Don's tribute to Lane. When Don came back to the agency, the only office that was available was Lane's old one and the Mets pennant was still in there.