Propaganda is meant to convey or reinforce an ideology. Since, at this time in history, there was a real “hearts and minds” battle between East and West, these kinds of campaigns can safely be considered propaganda. Similar postcards made by NASA at this time would certainly be propaganda as well.

A similar example in the West today is those ads for the US Army they sometimes show before movies.

JFK's famous "we choose to go to the moon" speech would be propaganda also.

I'll just add to what has already been said, that this is part of a broader campaign and is not just imagery used to promote the USSR's space agency. Also, literally every company in the USSR was state owned, so its hard for a marketing campaign to not be associated with them.

No, we'd call that propaganda as well.

It's not that "everything Russian is propaganda" it's that under the Soviet Union at the time these were produced there was a specific agenda and ideology intended to be propagated by the state and funded by it. And under that regime, if you made a holiday postcard showing a drunk Lenin and Stalin arm in arm staggering down a snowy street and tried to distribute it as "art" you would have likely ended up in a re-education facility. Context is relevant to the terminology here.

Also, checkout this: https://en.wikipedia.org/wiki/Whataboutism

If I had a penny for every time someone posted this link when it didn't apply...

This isn't an instance of tu quoque.

All marketing is propaganda, but not all propaganda is marketing.

Exactly my point. This is just marketing.