It’s more than fine to me—I think it’s rather sweet! And anyway, the way I see it, it’s got very little to do with me—why should I feel uncomfortable seeing some other couple be happy and affectionate?
At the same time, I would feel inclined to give them privacy if they started to, say, make out—and being unable to give them privacy might make me feel uncomfortable—but that says more about my societal conditioning than anything, probably.
Two side notes—thoughts that just crossed my mind—it’s interesting how we call them “public displays” as if someone giving their SO a kiss is more about its presentation to others than it is about the couple…. At the same time, why wouldn’t we expect SOs to interact with each other more intimately than they do with friends, acquaintances, strangers, etc.? .... What nosy, stifled people it would make us!