It's more subtle than that. Obviously no one who already smokes is going to say "Oh, the packet isn't as pretty as it used to be, guess I'll quit smoking now."
It's about the big, long-term picture. Companies spend money on branding and advertising because it works. You create the perception that your product is for a certain type of person, which makes them more inclined to buy it. By making cigarettes boring, you make them less appealing, and on average less people will smoke.
The proof is in the pudding. Social attitudes to smoking in Australia have totally flipped within a generation or two. It used to be something that everyone did. It's now mostly seen as a gross habit.