You should be concerned with this since it is an unwanted behavior no matter what the age. I start teaching puppies at 8 weeks that mouthing is not acceptable. I would personally do away with the kisses thing too since that is also a display of mouth contact. If you want the dog to kiss you that's fine but do it at a time when the dog has either learned not to mouth or is not mouthing.

When mouthing occurs you need to stop all interaction with the dog. Just simply turn your back and ignore the dog for up to 30 seconds. You can say "NO BITE" or some other command if you want but it is really not necessary. The dog will not like that you turned away and after repeated times of this will eventually put two and two together that when I do this, They do that and I don't like when they do that so I'm not gonna do it!!! The dog basically will realize that play ends when I open my mouth.

Realize that at 9 months the dog is going through a chewing stage. Make sure you have plenty of chewies, toys and whatever else you think appropriate around. I give my dogs lots of ice cubes during this period too!

You should see results pretty quickly if you are consistent!