Ahr river
I say go for it. If your world has it, add it.
Read my stories!
Obviously it doesn't hugely matter, but it's a good way to sell that something is normal if you have multiple unremarkable characters with that quality. But if you mean outright agender or otherwise nonbinary are accepted, you should be reasonably explicit about it, because dodging around outright mentioning it gives the opposite effect.
You will not go to space today.
Well, in this setting a majority of people idenifiy somewhere on the binary, so people who are agender/genderqueer aren't particuarly common. One of the characters in the main cast happens to not identify as a man or woman, but it isn't brought up particuarly often and when it is, it's often due to characters running into Prounoun Trouble.
Something doesn't have to be extremely common to be treated as unremarkable—people might tend to assume the wrong thing but when they're corrected their minds aren't blown away or anything. One or two minor characters is probably nowhere near approaching a majority of the total number of characters in your story, so it's reasonable to include them being explicitly whatever so the one agender character doesn't seem like a unique anomaly.
As for the character—I'd consider it pretty explicit if it doesn't go much beyond "he? she?" "no" "okay, they." Mainly I meant, describing a gender outside the binary as ambiguous is incorrect, if nonbinary is what you were after in the first place. Nonbinariness is sufficiently nonstandard to most people that going out of your way to avoid mention of it will essentially cause it to not exist in most readers' minds, and if you treat it the same as an ambiguous gender you risk that.
You will not go to space today.
So as long as, at some point or another, the character states they aren't a 'he' or a 'she' or something along the lines of that, it'll be decently explicit? But for the minor characters, should I make it explicit for them as well, or does the fact that the reader might go 'oh, this character is definitely agender, so this character could be agender too' okay?
It works the other way too—if a minor character's explicitly agender, stronger than any implication your main gives, then it helps sell the concept of that important character being agender. More characters having the same trait allows you to be less overt with any individual in general. But if everyone (who knows how that minor character identifies) refers to them with they or other neutral pronouns it can work; the most important thing, in a world where a nonbinary identity is normalised, is to avoid having people act like the gender is a mystery, which is what the Ambiguous Gender trope implies. I guess explicit wasn't the best term to use so much as, be consistent with the idea that this gender is a third category or at least a valid category outside the usual two, rather than just a failure to identify which category it's in.
(Caveat that of course it's possible for someone to just not want to disclose what they identify as, but—well, you should know that that's specifically what you want from them.)
You will not go to space today.
So I have a couple minor characters (they're probably going to be pretty much static and only have a few appearances)that I can't decide on a specific gender for them. In this setting, it's not completely unheard of to have people who don't identify as male or female. So should just giving them an Ambiguous Gender be a good choice or doe sit have an opportunity to be too distracting?