The design consists of the features of shape and configuration applied to a mattress with cover as illustrated in solid lines in the attached drawings.Drawings of the design are included wherein:FIGURE 1 is a front elevational view of the article to which the design is applied, namely, a mattress with cover;FIGURE 2 is a rear elevational view thereof;FIGURE 3 is a left side elevational view thereof;FIGURE 4 is a right side elevational view thereof;FIGURE 5 is a top view thereof;FIGURE 6 is a bottom view thereof; andFIGURE 7 is a perspective view thereof.