A good way to conceptualize this is to consider the fact that many things are organic aren’t ‘this’ or ‘that’. In other words, when we say an atom is sp2-hybridized, that is just our best description of the atom’s hybridization character.

In this case, we’re dealing with what are generally classified as imines - nitrogen-carbon double bonds.

In this case, you can resonate the pi bond. However, this yields quite an unfavourable resonance structure. The top nitrogen now has a formal charge of +1 and the lower nitrogen now has a formal charge of -1.

At first glance it might look like you could resonate, but the resulting structure is quite unfavourable. The top nitrogen would have four bonds and a positive charge, and the lower nitrogen would have two bonds and two lone pairs.

As with all resonance structures, they contribute to the overall electronic distribution in the compound. This resonance structure, however, being highly unfavourable - contributes very little. A stable resonance structure would be worth considering with respect to affecting the hybridization of the nitrogen atoms.

Thus, it would be inaccurate to say that the top nitrogen has sp2 character. Since it cannot resonate that pi-bond without gaining a formal charge (and becoming unstable), that resonance structure doesn’t really contribute to the hybridization (the molecule is best understood drawn as it is)