Yes, she is. I've actually talked about this few days ago.
Basically, Yunjin provides flat damage bonus (roughly +1100 NA damage bonus at Level 10 and 2000 DEF) which is minuscule on most characters because of their high NA multipliers. Think of Yunjin's NA like Crescent Pike's buff except it scales with her DEF. So, Yunjin's NA buff only works on characters with low NA multipliers such as Zhongli, Xiangling, Yoimiya, Ayaka and Keqing.
Because of Yunjin's passive which requires rainbow team comp, Keqing is naturally the best choice due to her 40 energy cost and little to no downtime on her DPS. If Yunjin has 2000 DEF, her NA roughly increases Keqing's NA by 60%-70% after taking enemy DEF and res into account.
In other words, Keqing no longer needs to use N1C combo when she is paired with Yunjin. Her N4C should have higher M/V than her N1C with Yunjin's NA buff. As a result, she will conserve A LOT OF STAMINA and deals more damage. Will Keqing be as good as Ganyu? Absolutely not. But will she be much better as a carry? Yes.