Garnfellow said:
These sorts of criticisms were plenty common back in the day, but Dragon would have been the last place you would go to find documentation of this. Not sure if the editors rarely published these sorts of D&D-griping letters (as opposed to Dragon-griping letters), or if the critics just didn't write to Dragon, or what. But there was certainly a vocal group criticizing D&D's treatment of monsters . . . for both mechanics and flavor. I remember the first time I read through some non-TSR publications, and it really opened my eyes. Not that I agreed with all of the criticism -- monsters were my favorite part of the game! -- but I remember being surprised that there were so many folks discontented with D&D. I never got that sense reading Dragon magazine every month.
But you would need to check out the APAs and magazines affiliated with other games to get a sense of the depth and breadth of the criticism. I can't think specifically of a letter picking on the ogre mage, but there is an essay in a Chivalry and Sorcerery supplement (I think the first Companion book) that really takes D&D to task for its monster design.
Garnfellow -- I know what you mean. D&D was mercilessly criticised, dissected and pooh-poohed in each and every aspect. I have seen it myself, read it in fanzines (well, fanzines from 90s Hungary, okay) and on message boards. But the criticism usually seemed to come from people who didn't like the whole of the game. People who decried it as "unrealistic" or "childish", people who thought it didn't simulate the kind of fiction or historical/mythical corpus they wanted it to simulat (most often Tolkien) and so on. I don't think it would have been wise at any time to cater to the wishes of these groups (just like it wouldn't be wise today to replace 3e with a reprint of OD&D, for instance, although I sure would get a kick out of it

)
I think Dragon is relevant because it was the kind of forum where you would find criticism from people who were fans, even dedicated fans, who nevertheless felt something was wrong and needed fixing. In short, a bit like ENWorld. So yesterday evening, I went through the Dragon archive and read everything that
a) had to do with rust monsters or ogre mages
b) was found in a reader's feedback or game designers' soapbox area.
There was no criticism. A lot of suggestions on how to use rust monsters as a DM or as a player (tame and train them for your army, for example

), but for a magazine which ran a lengthy debate on whether female dwarves have beards, or how fireballs
really work, the question was conspiciously absent.
Maybe I should also do a search of rec.games.frp.dnd, but there is no time for that now.