Yes, USB connectors are designed to maintain 90 Ohm differential impedance. USB founding consortium includes lead engineers from several connector manufacturers (Foxconn, Hirose, JAE), who actively participated in definition and overall construction of connectors, trading off the cost and electrical quality.
However, any non-coaxial connector introduces noticeable imperfections into transmission line. The impedance depends not only on relative configuration of conductors and their crossection, but also on quality of surrounding plastics, like dielectric constant and loss. So not all USB connectors are made equal even if they maintain the same geometry of conductors. USB connectors don't have reference planes, but they do have "signal return ground" in near proximity to D+/D- and Rx/Tx +- wires. In extreme case, a twisted pair of conductors does have a defined differential impedance even if there is no ground plane around, look at UTP CAT5/6 network cables.
Good thing is that the actual mating length across the entire connector is under ~20 mm only, while the signal wavelength even at Super-Speed rate (5Gbps, 2500 MHz) in surrounding dielectric is about 60 mm (and ~600mm for USB 2.0 rates). So the effect on actual waveform is somewhat alleviated if the connector impedance is managed within the 70-105 Ohms range given in USB specifications.