What do you mean by “cover the altar”? Do you mean the actual altar itself, or do you mean a display around the altar or in the sanctuary near the altar?
ON the altar itself: No. Absolutely not.
Some other place in the sanctuary: nothing expressly prohibits it.
From the GIRM:
- For only what is required for the celebration of the Mass may be placed on the altar table: namely, from the beginning of the celebration until the proclamation of the Gospel, the Book of the Gospels; then from the Presentation of the Gifts until the purification of the vessels, the chalice with the paten, a ciborium, if necessary, and, finally, the corporal, the purificator, the pall, and the Missal.
In addition, arranged discreetly, there should be whatever may be needed to amplify the Priest’s voice.