The scene was cut, she stood with a number of other senators (including Padme) to present a letter to Palpatine that discussed their distaste with his gained powers.
Now, just a handful of things about this scene that Kev created:
1. By the time of ANH, the Mon Calamari race would have been considered inferior by the xenophobic emperor. No way there would be one in the Senate. Ackbar was Tarkin's slave until he was freed during a rebel attack.
2. I'm not 100% on the time frame, so I could be wrong. I think Mon Mothma was run out of the Senate for openly supporting the rebellion before ANH. In fact, her homeworld of Chandrila was attacked because of what she said.