No one said Zoro isn't a Yonko commander. If some Yonko commanders are stronger than others, then it means some Yonko commanders are closer to the top tiers than others, meaning the whole "top-tiers won't look impressive against commanders" logic is BS. Zoro is barely outside the top-tier realm. He is one small power-up away from being a certified top-tier, and he packs enough power to murder about any character in the series. Of course, beating Zoro would make him look impressive.