Milquetoast is a very good diss. Almost as good as cockwomble. Although adding "of the highest order" to any decent slur gains massive extra boost. Given the prevalence of utter filth and crudity in modern parlance, which has somewhat weakened the power of such words, I have found that reintroducing antiquated derision carries greater impact. "Buffoon of the highest order", "gangrenous mendicant", "guttersnipe" and "utter nincompoop" are so unexpected these days that they slide in underneath the armour that has adapted itself to repel common or garden genitalia-related name-calling.
The round logo featuring our antagonist looks like it would go just fine on the top of an aluminium tin!
While we're on the subject of quality insults, the Germans have a few gems that approximately equate to milquetoast, such as Schwachstrom-elektriker (low voltage electrician, euphemistically), or Sauna-unten-sitzer (one who can't handle the higher temps of the top bench in the sauna)