compared with her sister.she is not so clever.

是被动，她跟她妹妹比起来她不显得很聪明。她是被别人拿来比的。

to be compared 被动

compared to 是compare to 的非谓语形式，一般用作状语。
而compare to 则做谓语。
compare to 的意思比较多，有和。。。。相比，如Compared to / with your classmates ,you are indeed very lucky.。还有把。。。。比作什么什么。如：Shall i compare you to the sunmer flower。