Mihawk isn't strong as Shanks
Mihawk bounty is inflated
Fighting Mihawk for years couldn't make Shanks to be a yonko candidate even
Beating Loki made Shanks be a Yonko
Mihawk stopped fighting Shanks when he lost his arm. After that, he became a Shishibukai with a bounty of 3,590.bi frozen, without being a yonkou. If Mihawk had been defeated, like Loki was, then Shanks could have become a yonkou. But Mihawk himself acknowledged that he stopped being a challenge without his good arm.
Now, Shamrock probably uses devil fruit as a supplement to be strong. Neither Shanks, nor Mihawk, nor Oden, nor Roger, nor Ryuma, nor Zoro need devil fruit to be strong, which lowers expectations for Shamrock's sword skill (haki included).