The problem isn't that we have seen nothing from Mihawk but the fact we did. It's not like with Dragon, where we literally never saw him engage in battle. Oda decided to specifically insert Mihawk into the action in Marineford. He was the one proactively launching an attack at Whitebeard, with Doflamingo questioning whether Mihawk is really gonna fight (rather than holding back), Mihawk was the one wanting to test Luffy's fate, only for Marco being confident sending Vista solo to stop him, Vista being casual and smiling while doing so and saying that the postponement Mihawk offered would benefit both of them. As
@ZenZu pointed out, Mihawk wasn't even singled out from the other warlords, and is now known as being partners with Crocodile hiding behind a clown on "empty bluffs island".
That's something you can blame Oda for. For a character that is supposed to be stronger than Shanks, Primebeard and Roger, the portrayal is horrendous.
Actually, We know more about Dragon than Mihawk,
When Mihawk appeared in Baratie, He flexed tiny bit of his Strength & Speed, which turned out to be just Gentle Blade & Strong Blade Mastery that anyone can learn, but Dragon? He blasted everyone with Strong Wind which is obviously Unique to him & whatever his Ability is, He also did it again in Sabo Flashback,
I also know that Sabo learned Ryusoken, indirectly revealing Dragon's Martial Arts, but can you name Mihawk's Sword-Style? We literally don't know anything about what Fuels his Swordsmanship. I also know who is Dragon's Father & i've seen him in Flashbacks, can you say the same about Mihawk?
If you ask People about Dragon, Majority will say CoC-Coated Ryusoken + Wind-Related DF, i'm not saying this is the case, but if it ends up being shown like this, will it be to the surprise of anyone? Not really
But Mihawk? We absolutely have no clue, except some far-fetched Theories,
Just because you've seen Mihawk test Luffy & Zoro, doesn't mean "Oh We've seen Mihawk Fighting & He didn't look intimidating like those other Legends", it's simply consistent with what He is built up to be, One of 10 Endgame Walls for SHs Dreams,
Do you know what is Bad Portrayal? It's when Mihawk shows his True Power & then you can show those Past Scenes/Statements & say but these don't make sense anymore? It feels like inconsistent,
You guys are not criticizing him for Fights He underperformed in, you're criticizing him for Fights He didn't want to participate in,
He ignored Krieg, He ignored Jozu, He ignored Ivankov, Daz, He wasn't interested in fighting Vista or Crocodile or Shanks ... etc
And tbh no matter what explanation we give, people will just continue to say "Oh but Marco sent Vista to deal with Mihawk",
They don't know the difference between sending Vista because He is what's enough to deal with Mihawk, and sending Vista because He is the best that's available regardless of his chances,
Vista can be used as Anti-Feat only if there were much stronger options but Marco was satisfied with only Vista, however that's not the case at all, He literally had no other choice,
Just like when WB gave his back to Aokiji & left Jozu to deal with him, was it because Oda wanted it as an Anti-Feat for Aokiji Level guys? No, it's because WB has no other choice