I wonder if it had to do with the lower energy level that affects everyone as they progress through the game. Most people go from a normal level of energy/enthusiasm to a depleted one. Maybe Maryanne went from an excessive level to a normal one.
Tony was like that, in his own way.
I'm sure not sleeping, being cold in the rain, and lack of food sucked out some of the energy. Also, Omar told her she played a sloppy social game early on, and she said she became aware of that at the fake merge. She may have simmered her emotions down some once she realized it, and that may have been why she mellowed some (her mellowness is equivalent to someone's normal energy).
I think Maryanne deserved/earned her win over Romeo and Mike. She was always very social, and she was liked by everyone (even though she got on some nerves being a motor mouth). She participated at camp by weaving palm fonds, gutted fish for eating, made Omar a mat, attempting to cut coconuts, etc. She always looked for an idol, and got lucky more than once. She helped get Omar out, although it is unclear whose idea it was since Jonathan said he and Mike thought of it first. If it was, it wasn't shown on camera. More than anything though, once she found the merge idol, she kept track of the combinations in her head that would for sure get her to the final three, and she didn't reveal her idol to anyone, and so it didn't make her a target. She knew Romeo wouldn't choose her to make fire. She read it right. Prior to the final tribal council, the jury didn't know about this, but had any of them known while still in the game, it could have blown up her plan. I also liked her self growth story of how she chose to not sabotage herself by playing her idol for Lindsay, even though she wanted to and was scared of losing the friendship. She said she decided to take the chance to lose (the friendship), because she deserved the chance to win. She is young and it showed sometimes, but she is also intelligent, and that intelligence shined brightly at tribal council.