I'd say "not necessarily" because jealousy can stem from so many differing causes. In a well-established, longterm primary relationship there can be more trust and better communication and therefore less need for jealousy, for example. Newer relationships that are still experiencing NRE can be more prone to some types of jealousy stemming from insecurity and uncertainty about the stability of the relationship.
On the other hand, someone who is comfortable with not having a primary may be better equipped to handle jealousy when it happens, and have fewer problems with this so it might "look" like they are less jealous overall. I think ultimately jealousy is more of an internal issue and less related to relationship style and structure than it is to how well a person knows themselves and handles stress, uncertainty and fear in their life.
*Edited to clarify that I have actually experienced both situations/poly styles (ie. as part of an established, longterm couple and as a "single" person dating multiple people but not having a primary of my own.