The -12V rail is considered legacy and unimportant by all modern ATX PSU manufacturers (source: am in the ATX PSU industry). They'll typically use a really shitty inverting boost circuit off the +5V rail. The ATX12V spec allows +/-10% voltage and up to 120mVpk-pk ripple, but it's not uncommon to see +/-20% and 200-250mV, even on $100+ PSUs, because no one cares. The only thing -12V is used for in a modern PC is the RS232 port, and most motherboards and serial I/O cards will have their own power supply for it. It also used to be used as the negative op-amp supply rail in some cheap sound cards about 10-15 years ago, but they stopped doing that specifically because the -12V rail was usually so noisy.
-12V is expected to be removed from the ATX12V spec in the next major revision, the way the -5V rail was removed in 2003. The only thing that kept it on there so far is that the PCI bus specified a -12V rail. But now that PCI is largely obsolete, and PCIe requires only +12V and +3.3V, the -12V is gonna be up for the chopping block soon.
The PSU is probably fine.