I know that you are a fellow Chadmiral enjoyer but I will answer this question since I’ve seen others ask this as well:
It is not possible for Oda to humiliate the Admirals in the same way that he humiliates the Yonko, since the Yonko is such an inherently meaningless and arbitrary title.
The rank of Admiral carries serious weight to it, you must literally be the strongest soldier in an army consisting of millions of soldiers. You can become a Yonko just by exploiting rumors that get out of hand and reputations that are not based on fact but are often based on rumor and misinformation. This is simply not possible for the rank of Admiral because the Marines are actively measuring the strength of their fighters.
You don’t become an Admiral by accidentally spreading lies about how strong you are, you become an Admiral by actually possessing strength. In that way, it is impossible for Oda to devalue the Admiral title unless Oda has the higher ups in the Government develop brain cancer.