is it social suicide to join a low tier sorority

No you sound like you shouldn't be in Greek life at all if you think tiers mean anything or should have any weight on your decision to join

In all honesty, AU is so small tiers really aren't a thing. Also, if you go in with that attitude you are going to ruin the experience for yourself.

Some people might look down on you if you join a lower tier sorority. You also might make close friends in a low tier sorority so if that’s worth it to you then join one. It’ll definitely lower you’re social status to some extent, especially with guys, but who really cares. If you’re in need of friends, join a sorority soon. If not wait it out and try for a better one.

Tbh you’re meant to go where your meant to go. I’ve only heard of the results sometimes changing in the spring but usually you get picked for the same ones because they are just the type of people your personality mixes with. The “lower” tiers are still great. Don’t sell them short. They are proud to be in their organization and have a friendship and philanthropy they care about. Literally the only thing that it would affect is mixing with frats but honestly that barely matters too. There are formals, sisterhood retreats, crushes, philanthropy and so many more things that make the act of joining a sorority amazing no matter which one. Do whatever makes you happy but if you think that rushing in the spring will change anything drastically from last spring I would adjust your expectations