It is never too early to start toothbrushing!
I actually started brushing my child’s teeth when he only had gums! Starting even before teeth emerge can help babies to get used to the sensations of tooth cleaning early.
She may also feel a little “itch” near where the tooth is emerging - a common occurrence during the teething phase. A gentle scrub with a toothbrush may feel rather soothing and enjoyable for her.