The ornamental design for a mobile phoneholder, as shown and described.
FIG. 1 is a perspective view of a mobile phone holder showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is an enlarged view of the selected portion in FIG. 1.
The broken lines shown in the drawings depict portions of the mobile phone holder in which the design is embodied that form no part of the claimed design.