While the Google Doodle for the Autumnal Equinox in the Northern Hemisphere is super cute, Google has the power to influence and/or encourage its users given that it's such an enormous platform. Therefore they should choose Doodles that advertise social causes and issues for their users to get involved in.
There are plenty of significant things they honor. If there is something that is more significant they can create another doodle for the same day. But the autumnal equinox is pretty significant, and even if it is a little fluff and not political or social, it is nice to have a doodle for it.
It's important to recognize and celebrate the changing of the seasons. Google is perfectly justified in celebrating the equinox today. Cultures depend on the changing of the seasons to break up the monotony of the year. It's fun, and there's no harm in it. I'm glad they chose such a doodle for today's Google homepage.
Google Doodle honors lots of serious things throughout the year. However, at the end of the day, it is still Google's site to recognize whatever serious or silly thing they want. Autumnal equinox is a hemisphere-wide event, and while it might not be super important to most people, it is still an interesting day.