I claim the ornamental design for a mobile stool, as shown and described.
FIG. 1 is an isometric view of a mobile stool of the present invention (the broken lines in FIGS. 1-7 are for illustrative purposes only and do not form part of the invention shown in FIGS. 1-7);
FIG. 2 is a side perspective view of the mobile stool shown in FIG. 1;
FIG. 3 is a front elevational view of the mobile stool shown in FIG. 1;
FIG. 4 is a rear elevational view of the mobile stool shown in FIG. 1;
FIG. 5 is side elevational view of the mobile stool shown in FIG. 1;
FIG. 6 is a top plan view of the mobile stool shown in FIG. 1; and,
FIG. 7 is a bottom view of the mobile stool shown in FIG. 1.