The ornamental design for a core of a mascarabrush, as shown and described.
FIG. 1 illustrates an isometric view of a core of a mascara brush;
FIG. 2 is a right view thereof;
FIG. 3 is a left view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is an enlarged top view thereof; and,
FIG. 7 is an enlarged bottom view thereof.
The broken lines showing the bristles and the cylindrical features on the proximal end illustrate environmental structure that forms no part of the claimed design.