STLINK V3 and Signal Integrity Considerations for a Debug Connector Which is Not Near the Processor
I personally use STlink's with 6 inch ribbon cables all the time, it works fine. STLink's are designed to be used with these. You may want to use a lower clock speed but you probably don't need to.
If you're really concerned with reliability I'd caution against using low quality dupont jumpers. Not because there's anything inherently wrong with them electrically, but because they break easily. The crimps get loose easily, and they're often made with low-quality wire that can get severed internally (usually at the crimp).