I think it depends on the kind of relationship you’re asking about and what you’re asking.
For example, I think Nepeta would be best to ask about compatibility for potential relationships or general questions about morail-type relationship. And in reverse, asking her about general romantic (matesprit-like) relationship could end up very biased and hit-or-miss (on whether she ships the pairing or not). But another example could be asking Kanaya for advice on auspistice-type relationships or matesprit-type relationships could work. Karkat could also work for matesprit and (especially) morail type relationships.
But If I could truly recommend, I would say work with whichever character you feel could work best. All characters fit in different ways and would give different opinions. But I would NOT ask any of the Alpha kids. They don’t know what they’re doing romantically with each other, and it ends up being harmful towards each other.
tl;dr: depends on situation. Nepeta, Kanaya, and Karkat could be good. Alpha kids could be really bad
I would suggest not asking Karkat for adivce advice, although his soosh-pap was succesful the actual long-term morailagience failed spectatcularily, and he is floundering in his other relationships. A good alternative might be the Sufferer/Signless, who had a love that went beyond the quadrants.
I would also not ask Equius or Eridan for advice.
At the end of the day, most of the characters are teenagers and teenagers are just known for bad relationships.
This is a good point that I forgot to make. And while many relationships did go horribly, there were aspects that did work (in some of them. not all). Never take complete advice, but on certain aspects could be really good ( such as Karkat’s shoosh-papping).