Well, it depends on what you consider Mother May I. To lean on your example of Murag and the fire.... the GM may deny Murag's player the ability to craft a fire based solely on the GM's conception of the region where Murag is. The GM had thought of it as a barren wasteland.
But Murag's player has the conception of Murag as a highly capable outdoorsman, who can start a fire under any conditions.
The GM decides not to let Murag start a fire.... that's MMI. No ill will, just a judgment call. Whether it's bad or not depends on who you ask.
If the GM allowed the fire or instead decided to let the dice determine it, then he's not allowing his perception of the way things should go to dominate play. It's not all up to him.
This is where we aren't going to agree I think. In my view, his concept is flawed. Is he going to expect to be able to start a fire underwater or in a vacuum? I don't think he is, so "under any conditions" becomes "under many conditions."
Now we know his skill has limits, so we go to the baren wastelands. If the DM has described it as having no brush or trees, then there is no wood. If there are no herd animals that roam the area, like Camels, then dung wouldn't be present, either. So that leaves if Murag's player said he was carrying wood(which is heavy) along. If no, then Murag's player is acting in bad faith, violating the social contract as well by attempting something that he knows isn't possible in that environment and expecting his concept to allow it anyway.
The DM saying no in that situation isn't engaging in Mother May I, he's preventing an abuse of the system. A concept ability doesn't allow the impossible. I'm all for giving a lot of leeway, but impossible is impossible and it's not Mother May I to enforce that.
But you can't always be sure everyone's enjoyed the game. Sure, they may have seemed like it, but maybe there was an issue and they just don't feel like it's worth bringing up.
Or maybe they enjoyed themselves, but could have enjoyed themselves more. There's no real way to know that unless you have an honest and open discussion about it... and that's not always easy to do. People can get very sensitive about a lot of this stuff, and that can prevent honest communication in a variety of ways.
I can see if they are enjoying the game. Even if there's a issue, that doesn't mean that doesn't mean that they did not enjoy themselves, which is the goal of the game. I think that even if someone could have enjoyed themselves more, that doesn't make the game a loss(from a goal of having fun perspective).
There have been many games that I have played in where there was some minor aspect or aspects that I didn't like, but the good/great ones dwarfed those and I had a blast. I don't have a right to expect the game to change for me. That's a very arrogant position to take as the changes I want can very easily be ones that another player or players find annoying or an issue.
If there is a minor issue, bring it up after a game and see why the game is played that way. It might be for a reason, or maybe it's not and it can change if the group wants it to. If there's a major issue(one that makes the game unenjoyable for you), bring it up right away or after the game depending on what it is, but don't get into an argument over it. If it's not going to change, find a new group.