The ornamental design for a mattresscore, as shown and described.
FIG. 1 is a perspective view of the mattress core with one mattress element, showing the design thereof.
FIG. 2 is a detailed view of a portion of the mattress core with one mattress element that would be generated by sectioning along lines A-A and B-B.
FIG. 3 is a top plan view of the mattress core with one mattress element.
FIG. 4 is a bottom plan view of the mattress core with one mattress element.
FIG. 5 is an end view of the mattress core with one mattress element.
FIG. 6 is a side view of the mattress core with one mattress element.
FIG. 7 is a side view of the inverted mattress core with one mattress element; and,
FIG. 8 is a side view of the mattress core with two mattress elements sandwiched together, where the mattress element depicted in broken lines forms no part of the claimed design.