Some characters get high K/Ds naturally, like Roses and Snipers (if you know how to play them). Chompers and Superheroes are hard to get a 3+ with.
It's also an indication of skill, but it's vague and arbitrary. You can't tell exactly how a person who showed you only their average K/D plays. If he went 30-0 on a full Turf Takeover game, it may seem impressive, but it's not at all. At high level you should be able to get at the very least 45+ vanquishes, even with low tier characters. You can do whatever it takes to preserve your killstreak, but other than getting the vanquish streak master boast, it doesn't mean anything on its own.