The ornamental design for a waterproof mobile phonecase, as shown and described.
FIG. 1 is a front perspective view of the waterproof mobile phone case of the present design;
FIG. 2 is a rear perspective view of the waterproof mobile phone case of the present design;
FIG. 3 is a front view of the waterproof mobile phone case of the present design;
FIG. 4 is a rear view of the waterproof mobile phone case of the present design;
FIG. 5 is a left side view of the waterproof mobile phone case of the present design;
FIG. 6 is a right side view of the waterproof mobile phone case of the present design;
FIG. 7 is a top view of the waterproof mobile phone case of the present design; and,
FIG. 8 is a bottom view of the waterproof mobile phone case of the present design.
The broken lines, and unshaded regions bound on all sides by broken lines, depict portions of the waterproof mobile phone case that form no part of the claimed design.