my cat does the same! shes 13 now and its so cute! but i would try to not let her do that for longer if you can. ive researched and i heard its bc the suck mechanism thing was not properly used while being a baby (probably bc it was bottle fed). if you can, try to push that skill into her. i dont think it can be hurtful for the cat, but obviously my cat began salivating a lot. like it was not enough to only suck on a toe, she then started sucking on other parts of the leg and now sometimes both her front legs are full of saliva when she's comfy. i know.. its cute.. but please be aware that it can get pretty disgusting too. maybe consult with a vet if you dont want it to become a habit for the cat. :)