Some folks slap the MMI tag on rules and design philosophies themselves. To those folks, it doesn't matter who the GM is.
And, honestly? I'm probably guilty of that myself. My back and forth with
@Snarf Zagyg is an example of this. And I'd be wrong actually. It's not an issue with the mechanics themselves. I do feel that certain mechanics lend themselves towards dysfunctional results, but, that's never guaranteed.
IOW, I think it's very important to keep context in mind as well as being very exact when talking about stuff. When I look at, say, AD&D, and talk about Mother May I in the mechanics, I should specifically point to specific parts of the game which, IMO, lead to dysfunctional play. A very simple example is the lack of any sort of codified mechanics for resolving out of combat tasks in AD&D. How far can your character jump? Well, it's ask your DM. Which can result to some pretty serious friction in the game between a DM and a player, both of whom are arguing earnestly and not trying to "get one over" on the other.
I mean, the whole "Can I swim in armor" is a meme for a reason. In a pre-Internet game, that question wasn't easy to answer and was pretty much entirely up to the DM's "gut" to answer. It totally depended on whose table you sat down at.
But, regardless of any specific answer at any specific table, if the table is content with the answer the DM gives, then, well, it's not really Mother May I at all because, well, there's no problem. I do think that when people point to rules and design philosophies though, it's not an unreasonable thing to say, "Well, the game gives largely no actual support in resolving this, and it's 100% dependent on the person running the game, which
can result in MMI play".
The problem is, people start typing too fast and omit that "can" part and state it as a fact. (I know that I am absolutely guilty of this)