True but...the word you might have chosen is 'diameter' not 'thickness'. While 'thickness' is correct, it is imprecise.
The tips of the blades, which is what does most of the cutting, on both the .9mm and the 1.5mm are identical. The length of the cutting edge, the sharpened edge above the tip, is obviously longer on the 1.5mm but either blade has sufficient length of cutting edge to cut most anything you could feed through the plotter.
The only functional difference is that the 1.5mm blade, and by extension the holder, is sturdier when is comes to resisting lateral forces put upon it when dragging it through heavier materials.