Murat's problem was that he was never viewed as a savior for the Italians, but rather as a guy who was put in charge, and was now trying to preserve his throne. Hence, he didn't get the grass roots upswing he needed to win, even when the outcome was in doubt. He had early success, but not enough to create the feeling of power. Ultimately, the neopolitans were correct: they had seen Joseph bonaparte installed and taken away, and they saw Murat installed and taken away, and then they saw the return of Ferdinand, and throughout it all the only thing that really changed was the ruler and the number of people getting killed. The lustre of "I am King, kiss the ring" was starting to dim throughout the world and people weren't just blind sheep who worshipped any old prince who happened to have the right connections to get installed.
Murat's goose was cooked when the Brits decided he wasn't going to keep his crown, which is why he went to war in the first place.
The best he could hope for is to have his kingdom and hang on for a while. He's not taking Sicily, or expanding much in any direction.