It also depends what you mean by 'mature'...
The N64 dynarec by Ari64 is pretty mature. However, N64 is a lot beefier than PS1, and it's pushing the limits of the Pandora's hardware.
Also, the N64 has a reprogrammable GPU, and most N64 GPU plugins use HLE. That means that you're not going to get perfect graphics emulation on all N64 games, and the gles2n64 graphics plugin doesn't support the ucodes for all games.
Future N64 emulation improvement will most likely come from improvements to gles2n64 or the addition of other gpu plugins. Porting mupen64plus 2, which is being discussed in other threads isn't going to really improve much.