Will it look bad to college if I did poorly in my junior year?

Yes. But what are the circumstances surrounding it? How bad is bad? What other activities do you do? What colleges are you aiming for?

These questions have to be answered in order to get a more detailed answer.

Bad as in straight C's. I don't do any activities for now. I'm aiming for UCI.

Straight C's is probably gonna hurt you for UCI. Are you a senior? What's your standardized test scores? Are there any external circumstances (family death, illness, etc.) that lowered your performance?

how about sophmore year and like the first quarter of junior? T_T i am aiming for UCI too

^ It's gonna hurt you, unless you have a really good excuse. [Family member died, Diagnosed with Cancer, etc]
UC's only look at sophmore+junior year.

Junior year is said to be the most important year in high school.
Getting C's and having no extracurricular activities will most definitely hurt your chances of getting in.
Like what xVivskies said you have to have a good excuse to why your grades went down. If it's because
classes just got harder or you didn't "feel" like studying, then it's very certain that colleges won't admit you.

Just raise your grades and look into extracurricular opportunities. Join clubs or volunteer at a local hospital.
You want to have a resume that stands out from the rest because you're basically selling yourself to the
colleges. Study hard and do well on your SATs!

Good luck!

-Mimsy