The ornamental design for a decorativelantern, as shown and described.
FIG. 1 is a perspective view of a decorative lantern, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines showing the plastic shade and support in FIGS. 2-5 and 7 are included for illustrative purposes and form no part of the claimed design.