Hi Adam and welcome to the forum...
I would suggest the only 'bad' reading you can get for prospective fault current is one caused by poor test connections. The prospective fault current is what it is, there isn't really a good or bad value for it as it will depend mainly on the distance from the supply source (most typically the substation supplying you).
That being said, it is possible to get weird readings.
Remembering that the PFC is derived (i.e. you use the highest value) from the Prospective Earth Fault Current (PEFC) and the Prospective Short Circuit Current (PSCC) it can reveal things about the installation which could be one interpretation of a 'bad' reading.
So consider a TNCS earthing arrangement on a single phase 2 wire supply (typical domestic arrangement). If you get a PSCC of say 0.865kA and a PEFC of say 0.125kA... what could that be telling you?
If you got similar readings from say a TNS earthing arrangement, would you be concerned? What about a TT earthing arrangement?
The only other thing that you could consider to be bad might be if you get a reading of say 6.5kA... your house is right next to the substation for the entire estate and you're on a TNCS earthing arrangement. Why might that be bad?