The point of that match wasn't to show how weak Smoker was against a haki user, but how varying levels of haki in general effect a match. Prior to that we hadn't seen different levels of Haki, so the point of Vergo was to illustrate that. Also, it was mid not low diff.Smoker's ability was very dangerous when none of the crew knew Haki.
Vergo showed just how weak smoker is when faced against an opponent well versed in armament haki.
And lets not forget the 3 beating he got that arc: Law, then Vergo, then Doffy. I mean WOW.
Law had the element of surprise on his side with a complex DF to learn from. Smoker deserves credit for lasting as long as he did against him, learning as he fought.
And Mingo? There's no shame in losing to him, especially when he was injured. Mingo is way stronger than Smoker anyway, and he still put the works on Luffy.