I'm going to tell my partner she's depressing on our commute
What is your reason for wanting to tell your partner this?
No, there's no reason. She just likes to depress me all the time.
Calling someone depressing is a pretty rude comment to make. If you think your friend is dampening the mood then it may be a better idea to find out what is potentially bothering them.
I'm bored and her depression is increasing. I feel like my friend is hopeless.
Instead of calling her depressed, ask her why she might be feeling down. Listen to her and prod her for reason as to how you might be able to help. Depression is serious and sometimes they just need an outlet to vent to.