I have a planted 10g quarantine in a south-facing window that gets quite a bit of extra light from sunlight. To compensate, my artificial lighting is reduced on this particular tank.
Some even encourage supplementing artificial light with natural sunlight. It's not bad per se. Like all things planted tank, it depends. It could potentially be a problem if it puts your total light levels into algea growing range. Whether it does for your particular set-up depends on a plethora of other factors, so adjust accordingly.