Is Catherine a prototype Manic Pixie Dream Girl? The Wikipedia page lists her among their number, and as tvtropes.org says "[the MPDG is] stunningly attractive, energetic, high on life, full of wacky quirks and idiosyncrasies (generally including childlike playfulness)" - check, check, check, check. Additionally one might reasonably notice that the film is called "Jules et Jim" rather than "Catherine" - is she just something that happens to the two friends (perhaps even " bros") rather than a fully-realized human being in her own right?
It can't really be said of course that she exists only to serve and save the male characters, as her influence on their lives will turn out to be a rather destructive one, but neither I think is she fundamentally independent of the male characters, many of her actions being predicated on (sometimes pre-emptively!) revenging herself on them. Jeanne Moreau is of course brilliant and breathes a lot of dimensionality into what we see on screen but that may or may not be anything to do with the script.
Truffaut is a great director, able to be very playful without it starting to grate in the way that I find Godard sometimes does, and this is a great piece of cinema by anyone's standards but I do wonder what we should be taking away from it. Hopefully not that chicks will really get in the way of a good bromance; nor that men should find themselves a Gilberte who will dote on them completely no matter how sh*ttily they get treated. Maybe we should just stick with "polyamory is complicated, don't enter into it lightly or you might end up getting burned".