PLOTMF uses PLOT, so you need to change the LineWidth of PLOT using the numerical output of PLOTMF [xOut,yOut]:

a = readfis('tipper');

[xOut,yOut] = plotmf(a,'input',1);

plot(xOut,yOut,'LineWidth',10)

Thank You Mr. Martin Trauth !

Thanks Martin H. Trauth

you can also use:

plotmf(fis,'input',1)

Then when you get your graph, go to (Edit) then (Figure Properties) click on each membership line and go to (Line) below and change the width from (0.5) to any value you want