There are deifferent definitions of the word "prayer" for Catholics. One is worhipping God. Another is to ask saints to intercede for us before God, just like you'd ask a friend to pray for you.
We don't say a prayer to someone other then God.
Moreover, asking someone to pray for you is far different then expecting Mary to be Omniscient and hearing all the the requests made to her (as God hears those made to Him)