Of course it's an accomplishment, even if others did it long ago. If I successfully run my first marathon (unlikely), it'll still be an accomplishment even though someone else first did it 2500 years ago. Getting a spacecraft into Mars orbit takes skill and careful work. I think it clearly warrants congratulations.

Also, by the way, the first Mars orbiter wasn't in 1965—that was the first successful flyby. The first successful orbiter was Mariner 9 (https://en.wikipedia.org/wiki/Mariner_9) in 1971.