I say do it in cotton if you want to! It's a placemat for sheep's sake! :D Using similar weight yarns would be an issue if you were knitting something to wear, but since the only thing this will do is protect your table, it doesn't matter if it ends up a little bigger, or smaller (unless it's smaller than your plate, then you have a problem). Go for it, if cotton is what you want to use. the only POSSIBLE thing I can think of is it might have a different reaction to heat; so don't stick the placemat under your fondue pot.