應該不會比較好
You looks pretty before I think!
(這是哪裡學來的英文?)

"You looked younger and prettier than I imagined."
或許因為前面你有其他的句子描寫你的不期而遇(when I saw you balabala...)
而動辭已經是過去式了
那一定得前後一致

否則,上面Jorja所寫的
"You are younger and prettier than I thought (imagined)."
是比較好的寫法

另外,加個even,我想女孩應該會更高興沒錯