I find it interesting that your question was originally posted in the general "Literature" group on eNotes just because it's absolutely impossible to attribute the accents of actors that don't exist on the written page of a play. As a result, I went ahead and put your question into the On Golden Pond group in hopes to get traffic from people interested in both the play and the movie(s.)
Here arises yet another problem: there are two movie versions of On Golden Pond as well! The most famous one stars Henry Fonda and Katharine Hepburn while the most recent one stars Julie Andrews and Christopher Plummer. I remember distinctly that Charlie in the Henry/Jane Fonda version has a very strong Maine accent, so I am assuming you are speaking about that particular movie.
That being said, you are right to think that it is only Charlie that has a Maine accent. Katharine Hepburn, Henry Fonda and even Jane Fonda and Dabney Coleman simply sound like "themselves" in this movie. They all have distinct voices and did not alter them (at least to my amateur ear) for the movie. This is why Charlie's strong accent tends to stand out.
In regards to what accents they do have, it's again almost impossible to distinguish. Yes, the Thayers are supposed to be from Delaware, but I can't detect any significant Delaware-esque accent in their voices (despite the many oscars won for these different roles.) Henry Fonda is originally from Nebraska but spent much of his time in New York, where Jane Fonda was born. As for Katharine Hepburn, by the time she did On Golden Pond, her voice was absolutely affected by a tremor she developed later in life and (kind of) became her calling card in those later years.
In conclusion, your question made me want to call upon the soul of the fictional Henry Higgins from Pygmalion to be able to figure this out even better. Alas, he is only a fictional character as well!