Booping noses means to rub together your noses as a sign of affection.
And sweet nothings are the meaningless sweet words of affection and love you'd say to your lover.
This sentence tries to say that if you are a couple who does that in a public place to the extent of making others uncomfortable, then probably you deserve to be ridiculed.
Edit:
@Inglish Teeture is right about the exact meaning of "you deserve to get breadcrumbs thrown at you". That meaning somehow didn't occur to me :)