I am curious of the typical temperature of a nuclear fission reactor in a NTR and I heard that increasing temperature could increase Isp but couldn't find any useful data online. Please provide viable answers thank you.
In general, the temperature is as high as they can get away with without melting, eroding, or weakening the reactor core -- this is also the motivator for unusual reactor core styles such as particle beds (in which the uranium is in little ceramic or metallic kernels which are in a bucket).
This is because the achievable Isp is directly dependent on temperature. Rocket nozzles essentially convert thermal energy in the form of temperature into directional kinetic energy of the jet.
- Small Nuclear Rocket Engine, 2015: 2726 K
- NERVA in the late 1960s: 2272 K
- An ambitious high temperature design (2019): 3300 K
NERVA is the original solid core nuclear rocket and probably the one which has seen the most realistic testing (though never flown). Since that time, there have been numerous attempts at modernized nuclear thermal rocket designs, which have pushed the temperature upward. As a result, achievable Isp has risen from around 700 (some of the lower-performance NERVA tests) to the 800s (baseline for NERVA) to 900 or even 1000+ (especially if you believe the more optimistic speculative proposals).
If you want to take a deep dive into some of the finer points of NTR reactor design, you might want to look at this.
Even greater Isp requires approaches to heat propellant hotter than the reactor's melting point. This can include gas-core nuclear rockets (in which the reactor core is supposed to be melted (and vaporized) with uranium plasma confined either by gas currents or by an internally-cooled quartz "lightbulb", or "nuclear electrothermal" concepts such as the one asked about in this question where electric power extracted from the reactor is used to drive an arcjet-style "afterburner" at the cost of lower thrust, higher complexity, and heat radiators rather than no heat radiators. Even higher Isp than that those achieve (up to around 3000s) would require fusion rockets, the deeply questionable nuclear saltwater rocket, or the questionable-in-a-different-way Project Orion type.