One could argue whether or not Donna Reed gave a good performance, but the bigger issue was the fact that she reinvented the character of Miss Ellie. That's fine if you're doing a play, as performers are allowed to put a new spin on a static story. However, a serialized TV show is a completely different beast. Recasts take people out of the story when the replacement actor is drastically different. Any recast would be hard to accept, but casting a "BBG type" would certainly have helped with the suspension of disbelief.
Nancy Olson looks like she fits the bill to me. Of course, if they were going to go with a sitcom mom from the 50s, I think Barbara Billingsley would have been a better choice.