It's interesting that you mention alts. To clarify my earlier statement, the achievement point number is not meaningful. A low number doesn't mean a person is inactive or new; they could be on an alt. You can't draw any meaningful conclusions just by looking at a person's achievement points. How many people even have an idea of what "a lot" of achievement points looks like? How many achievements does 13,000 points represent? What quality of achievements or difficulty of achievements does 13,000 points represent? It's just not worth taking the time to put that information on an adventurer plate.
However, more plate elements to showcase specific achievements would be meaningful. If, for example, you saw someone with an adventurer plate customization that represented completing all sightseeing log entries, you could draw some conclusion about how that person enjoys the sightseeing log; or someone with a plate customization for getting all caster jobs to Lv100 might be signaling that casters are by and large what they prefer to play. Specific customization elements for specific achievements can tell you something about what activities the player enjoys and what accomplishments they're proud of. That would be way better than just an achievement point number.