Well a GPON frame is 0.125ms. So the best jitter is ~0.125ms, but even if the entire upstream segment was saturated you'd be looking at probably 1-2ms at absolute worse case. The OLT will not allow one user to hammer the upstream badly like you get with DOCSIS.

So it's really a non issue (XGS-PON is even better as more data per frame means heavy upstream 'clears' the frames quicker), IME consumer routers add more jitter themselves even with ethernet on the LAN side (and WiFi is a lot worse).