Do you think an old (fictional) diary extract would make a good prologue for part of a Sci-Fi novel?

Absolutely! It adds a personal touch, especially if your story is written in third-point perspective. Sci-Fi novels often have a tendancy to get very... unrealistic and slightly hard for modern-day readers to connect to. If your extract is from a time very similar to ours, then all the better! But it doesn't have to be, although I highly suggest that it is more relatable to common modern times. Good luck!