Is it acceptable in modern Russian to address a person as "господин Смирнов" / "госпожа Смирнова"? Or does that sound weird, outdated, or perhaps condescending?
Short answer: It is fully equivalent to Mr./Ms. nowadays.
госпожа is a modern polite address, to be combined with the last name.
A possible polite address is also first name + patronymic if you know it. It's more polite, but also somewhat closer. Not a clerk to a customer, but business partners to each other.
It was not acceptable in the Soviet times because of the "bourgeois" connotations, as it was the address in tsar era. Soviet times had
товарищ (i.e. the over-used "comrade") and
гражданка ("citizen", used rather in unique circumstances when the addressed person "was not a comrade", i.e. that how a coroner could address the murderer suspect).
Still somewhat in use is
гражданка, as it has no "communist" connotation, although quite special, even if not in the above sense. When in doubt, use
To call out someone on the street ("Hey, you!", "Excuse me!", etc.) use
Девушка, вы обронили билет!
господин/госпожа are for sure acceptable, though one has to keep in mind - that these words are extremely official.
So during common conversation you should avoid these words.
Господин/госпожа applicable in official messaging or if you want to address someone extremely officially, e.g. Mr. Petrov, we are going to arrest you/г-н Петров, мы собираемся арестовать вас