Tell the people that supplied the QR code that its quality is insufficient. Ask them to provide it as a vector image or a higher quality raster image. You cannot make diamonds from dreck.
I often recieve QR codes as RGB .png files at 72 dpi. It is preferable to get hi res or vector, but when I can't (client can't/won't) I have uprezzed them in grayscale/bitmap without problem as they are grids. Just do not rotate them. Keep them plummed Vert/Horizontal....
And alway double check with your smartphone/ipod touch/reader...
