Dribbling cats are a common phenomenon. That does not mean to say that dribbling cats are desirable. If your cat is old, has never routinely dribbled in the past or its dribble becomes smelly then there might be a problem with its teeth. Rotten teeth and abscesses are a big problem for older cats. Vets now advise cleaning a cat's teeth but this is easier said than done. Some vets will clean teeth under a general anaesthetic which seems a slightly over the top procedure given the risks of any aneasthetic. Most dribbling cats do so while they are being petted. The dribbling is usually associated with the characteristic treadling of their paws. The treadling is an action kittens do while feeding and they dribble just as we do at the prospect of food. Somehow some mature cats never leave this behaviour behind. The most obvious link with early feedin is witness in jumper suckers, those cats who literally suck at their owners clothes as if there was a nipple to be latched on to.
Cats do dribble sometimes, but yours seems to be doing it rather a lot. If she is in no pain that's good, but there is the possibility of an underlying infection or a problem with her teeth. If you look at this website on first aid for cats and go to p.14 (mouth problems) you can check if your cat has any of these symptoms.