The ornamental design for a side element of a shoe upper, as shown and described.
FIG. 1 is a perspective view of a side element of a shoe upper showing the new design; and,
FIG. 2 is a side elevational view thereof.
The uneven broken lines surrounding the design element represent the bounds of the claimed design. The uneven broken lines in all figures showing the remainder of the shoe are for illustrative purposes only and form no part of the claimed design.