I am using clangd vscode extension on a project and everything is working like a charm.
But I have an 'OCD Like' configuration I would like to configure.
I have the following struct definition:
Is there a way to change how clangd shows the preview index?
I would like it to show with two digits [00], [01] instead of one [0], [1] on lines 10 and 11.

There is no current option to configure formatting the designator hints like this.
You could file an issue at https://github.com/clangd/clangd/issues proposing to add one.