Ukraine's Slava could serve as a flagship only if made operational

Would it have made sense for Ukraine to use its inherited Slava-class cruiser as the navy's flagship?

Only if the cruiser had first been made operational; in its actual unready condition, Ukraine's frigate was the more realistic flagship.

The unfinished Slava-class hull offered symbolic scale but no immediately usable command platform. Operational status is decisive here: a smaller frigate that can sail and support the navy is more useful than a prestigious cruiser that cannot.