Alignment for complex HTML elements can be a very tricky issue, and it largely depends on the container and styling it has. The plugin has options to add alignment classes to the rating block, but, these classes don’t include actual styling and they are meant to be styled individually for the theme used.
To center element, that element should have the width set. But, if you set width for the rating block, that block will no longer be responsive. The plugin automatically makes stars smaller for the rating block to fit the current screen with. Having fixed width set for the rating block will prevent the plugin from making it responsive.
You can use CSS for responsive breakpoints to set different widths, but that too depends on the theme (themes usually have different breakpoints and different grid systems), and it is best to style rating block alignment for each individual theme to make sure alignment works for your theme. Also, depending on the theme and the grid system use, there might be some other way to center elements.
Plugin alignment feature is only placeholder class name, styling must be defined for theme.