Without knowing the context behind the sign stating this, it is difficult to say if it is a common view.
Did they mean that Satan is the Lord of this world because the majority of this world have chosen to follow him? or do they mean Satan has ultimate authority in this world? Most Christians would not object to the first view, but the later is uncommon. As you said, Matthew 28:18-20 shows that all power was given to Jesus.
Ironically, if Satan has ultimate authority on earth, how are we to "be careful" as the sign suggests when we are completely defenseless against him?
In the Bible, it is written that we cannot serve two masters (Matthew 6:24) - either we serve God or we serve Satan. True obedience to God is difficult, because when Satan comes to us, he comes home. In our natural sinful nature, our enmity is against God. However, God has promised us from the beginning that He'd put enmity between us and Satan, between the woman and the serpent (Genesis 3:15). This was the plan of salvation set from the foundation of the world. So to all who will to serve God by grace through faith, it is possible. We are not the "god of this world", but we are the adopted sons of God through Jesus.
Submit yourselves therefore to God. Resist the devil, and he will flee
from you. (James 4:7)
This is easier said than done, every willful sin must be overcome. So God allows us to enter into trails, into open warfare against Satan, so that we can see what is in our hearts and be humbled, receive His preparation, and win.