This is where the GM’s conception of the setting is a huge part of it. If you decided “these elements are too silly or anachronistic for my super serious setting” then you’d likely have shot them down.
Which is something I think many folks posting in this thread may well advocate, though I can’t say for sure.
GMs being precious about their setting… their world… is a big part of many MMI issues.
A few thoughts in response to these posts.I wouldn’t say never, because there are always exceptions. But I feel like such a decision is not one to be made lightly. If something hasn’t yet been established in play, then it’s possible. The GM should consider such suggestions seriously because this is literally the player saying “I want X to be part of play.” So saying no to that should have some strong reason behind it. More than “I didn’t imagine court jesters as being a thing in this world”.
At the same time, the players should allow the setting as it takes shape to inform their choices and desires.
Since the example @pemerton gave was from a one shot of In a Wicked Age, and early in the session when they were making characters, I don’t think the player requests were going against anything that had been established. At least not beyond the loose genre and name lists offered by the game as a default.
Also that the players were kids matters as well, I’d guess, and these kind of social considerations may be quite important.
I think with many RPGs, the setting can be held as paramount. But I don’t think it should be so, at least not as a default.
First, I agree that playing with kids makes a difference. I think it makes much more sense to go with what they're imagining, than to open up needless, and perhaps needlessly rancorous, discussions about what we should or shouldn't be imagining together.
Second, as you say it was a one-shot. Why waste time in a 2-ish hour session debating the garb the illusionist is wearing, or whether the brutal warlord has a Bond/Powers-esque cat?
Third, In A Wicked Age begins by drawing four playing cards and consulting the "oracles" - a table with a series of pithy entries describing trope-y, and thematically laden, people, places and events. Then the participants all go around the table, identifying characters implied (whether through express mention, or more obliquely) by the oracle results. Then the players choose a character each to play, and the GM gets the rest as NPCs. There's also a list of example names, although in our session I was the only participant to take names from that list. The others just made theirs up.
So there is no setting in the FR-ish sense, independent of this process.
Fourth, a reason why preciousness about setting can produce "Mother may I" issues is because it is often assumed that the GM will use their knowledge of the setting as a basis for adjudicating player suggestions, including gating things like costume and pets (and other higher-stakes things too). In A Wicked Age doesn't rest on that assumption - for a start, as per point three just above, there is no asymmetry of knowledge between GM and players!
The GM is expected to frame scenes, and to say what their NPCs are doing. The players say what their PCs are doing, and this can include introducing coherent content into scenes too (eg Romulus's player, when she wanted Romulus to kill someone, introduced his "wall of axes" from which he took one to do the killing - this seemed to cohere well enough with Romulus being a tyrannical warlord in his camp). The game only moves from this sort of consensual resolution to mechanics when too characters come into non-verbal opposition (fighting, or one hiding or running from the other, etc).
A 5e D&D GM who wants to reduce the degree of "Mother may I" in their play could consider some of these points and how they could be applied in 5e D&D. 5e D&D is more prep-heavy than In A Wicked Age, but it doesn't need FR-level setting stuff to work. There's no reason why players can't introduce elements like costume, social conventions, etc. And there's also no reason why players can't at least suggest coherent content for scenes, for PC's knowledge, etc. If the GM doesn't want to simply say yes, they can set a DC and call for a check!