Let's look at facts (as stated in the manga) that we know about Mihawk:
A) His claim to fame was his fights against Shanks. Those fights happened before he lost his arm and before/along the time he got his scar from a base BB. So the legendary battles are against a shanks that was not able to escape from Lord of D. Coast in time and saving Luffy.
B) He never showed enough mastery of Haki to hurt top tiers. We know he has a black blade and knows haki, but even his attack meant to test his strength against the top was not strong enough to cut jozu and the black blade alone is not enough to break Croc Hook. A hook that pre-ts Luffy broke with ease.
C) He stated on panel, before attacking Luffy, that he would not hold back because his blade didn't know restraint. Pre-ts Luffy was fast enough to evade him. Later Luffy escaped him with relative ease (in return to Saobody we saw that his speed was not comparable with earlier).
D) He was able to cut a huge ice block. Overall his best feat. His best feat is being equal or slightly lower than dressrosa Zoro vs Pika.
E) Vista was considered enough to hold him. On panel it was show that he had enough technique to hold the WSS down without taking a single scratch. The battle ended when Mihawk asked for a time out. You may think that Vista was a top tier but later we saw him fighting Aikanu along with other commanders. A couple of minutes with Akainu did more damage with him than facing Mihawk.
F) After the timeskip, the WG decided to capture him. On panel, only fodders are sent and deemed enough for the WSS.
TL: DR: Using only feats that Mihawk has show on panel he looks about on par with commanders and even slightly weaker. His biggest claim to fame is having a black blade but this blade does not seem to give him enough of a edge to win with ease against top tiers. Overall, he's a fraud.