Do I have to do this "Go to Acceptance Criteria and set them to default" every time I use the optimizer?

You can use "Set to default" command in Acceptance Criteria page to reset the criteria params to their default values. It does only that - rests the params. If your Acceptance Criteria were not changed or you don't wan't to rested them, you do not need to use the command.

Optimizer changes the strategy parameters and if it finds a better strategy, that complies with the Acceptance Criteria, it shows it.

Optimizer cannot make every strategy more profitable. It depends on the strategy indicators, parameters and the initial stage when you start optimizing.

If Optimizer does not finds a better strategy, you may increase or change the parameters limits - Min and Max values. Probably it can find better values in the new limits.

Sometimes, it works and at most time, there isn't any difference in the final outcome from the original.