if by a change in mixture you mean the fuel:oil mixture, yes .. that could affect idle.
At idle the air:fuel mix is rich, about 8:1. If the idle situation were already borderline, this rich mix in combination with a richer (more oil) fuel:oil mix could foul the plug. Other influences could be spark plug heat range and plug gap. A cold plug and small gap would be more prone to foul and kill the spark.
Higher humidity and hotter air temps also tend to richen the air:fuel mix. (Water vapor displaces air. Less air per volume of air richens the mix.. Hotter air is less dense and richens the mix for the same reason.. less air per volume..)
So, if you've got a carb with an air mixture screw, try leaning the idle air mixture a bit .. elsewise, if you've added more oil to the fuel mix recently, cut back to where it was.. or try a combination of both.
Adjust oil according to specs or to synthetic oil-mfg recommendations. (Those who've got an oil-pump that can only deliver one set fuel:oil ratio should use only the oil type recommended in the owner's manual.) For conventional oils, no tail-pipe oil drips and little or no smoke is pretty safe.
If a top speed plug chop (use a brand new plug) shows definate sooty residue (unburned fuel residue) on the plug, try a hotter plug. Set the gap to factory specs or a little wider if there's a lot of miles on the engine..
But if the bike's been modified with a different carb or main jet or other changes then all bets are off.. you gotta get down and tune it more carefully.. the general tendancy is to jet too rich where both low RPM performance and peak power suffers.