The Peter Principle would imply she was a good home secretary. She was actually kind of a disaster, but spent her days keeping out of the press. It's fairly clear her instincts remain to disclose as little information as possible to the press. It's less clear that's a good idea as PM.

Edit: She was, however, probably the best candidate the party had. Ironically, the Labour Party has at least five people in parliament right now who'd make a better Prime Minister, but a fair few of those aren't even in the shadow cabinet.