I do not think that anybody has mentioned anywhere that she is God. No, she is not God and therefore not omnipresent.
Have you considered the way prayer works? Have you considered why St Paul was always asking his converts to pray for him and his fellow workers in the vineyard of God? Unity of purpose in prayer yields answers to petitions. He prays on his own, off course, but he understood the importance of unity in prayer or joint prayer. Was Paul present with each of the different churches at the same time they were all offering prayers on his behalf? or rather, were they present with Paul, all at the same time? Yes, spiritually, not that Paul or they are omnipresent.
When we truly pray ( not with wondering mind while we seem to be praying), we spiritually partake in heavenly things. right in the presence of God, his angels and saints. We unite in Spirit. Same thing that happens when we are continually walking in the light or in the Spirit. She does not need to be everywhere to hear us pray or join in our prayers. For, we have come unto Mount Zion, unto the city of the living God, the heavenly Jerusalem and to the innumerable company of angels. To the general assembly and church of the Firstborn which are written in heaven and to God, the judge of all and the spirits of just men made perfect. And to Jesus the mediator of the new covenant and to the blood of sprinkling that speaks better things than of Abel. Hebrew 12:22 -24. When we commune with God with the right frame of mind, irrespective of where we dwell, even if all are praying at the same time, we are in his presence with the heavenly hosts. The RCC pray to God through our Lord Jesus Christ. They ask her and the saints for intercessory prayers. They understand Jesus is the only way and their prayers are directed through him.
In John's vision about the heavenly throne, he saw the importance of the prayers of the saints before the throne of the Lamb. Their prayers were so precious that they were placed in golden bowls (Rev5:7-9). That is, the prayer of those who have died in the Lord, together with the saints who are living in the world are like incense before the throne of God.
In Rev 8:1- 4, we see another vision of St John. He saw the prayers of the saints being offered on a golden altar before the throne, He also saw that the prayers of the saints were presented to God by angels who stood before the throne. Before the Lamb and God the Father, were seen the prayers of those who believe in Jesus Christ (both the dead and living) as giving glory to God.
If we can ask our Pastors or fellow believers for prayers, why can't we ask her or any of the saints? After all, when she was on earth, she interceded for people at the wedding at Cana and her Son obliged her. Why do you think he will not oblige her again?