in the show's timeline, Clarke has known Madi the longest, aside from her mom.

Her friends went up to the ark for six years(?) and in that time, Clarke met Madi after 42+ days alone, in the valley.

Clarke only knew her friends in at most 6-8 months timespan, if I understand correctly from other posts.

They did not know much of each other before they went to Earth as they were from different stations and Clarke was in solitary.

it's understandable how Clarke cares more about Madi. The only issue was the show's execution of showing this relationship to us the viewers. It was not enough time to understand their bond.

But yeah in the end that's how they wrote her character. Total mess.