I think I'll defer my opinion to own Mr. Hobbs on this one. He's an authority on both movies
It's a conundrum. She's definitely pure evil in The Natural, although psychologically interesting. Almost like Estella in Great Expectations, in a way; she's been conditioned to tear down greatness.
Myra... well, she's a bit of a shrew at first, but heck, Dale is a little crusty himself. They both want what's best for the boys. Now, one could argue that she's a bit of a bandwagoner (if they had pink hats in Hickory, she'd be wearing one), but since Coach Dale ultimately gives her the seal of approval, she's okay in my book.
So, that leaves it as a coin flip. Plus, who knows, if not for Harriet Byrd, Hobbs might have grown into a cocky superstar driven by his ego. Who knows, he could have turned into the next Barry Bonds. Instead, the career-crippling injury kept him humble, and led to one hell of a fine story.