Assuming that we have a representative range of skin color and other phenotypical features, how do you all feel about how common and integrated that should be?
I mean, I think there's a difference between saying that your halfling can have an epicanthic fold, but that means they have to be from the far eastern parts of Smackdabmiddleearth, or that feature just being common enough in the core region of the campaign/setting.
So in other words, yes, the character can look like the PC, but they're "exotic". Is a general "melting pot" situation preferrable?
I don't think it's a matter of 'preferable' personally. If you have established for whatever reason before that the prevailing skin color among wood elves is greenish to bluish, and someone wants to come in and play a black one, then you might have to come up with an explanation for how they are from a distant part of the continent, or something. If you have never really addressed skin color in a restrictive way before, you have a bit more freedom to say 'oh yeah it's always been like this.'
I do think it is reasonable for some races in some settings to be a little more restrictive. If someone asked to play a dwarf in my campaign with Native American features, sure. If someone asked me to play a drow but with Native American skin tones and features....well drow are, for better or worse, pretty tightly defined in their visual appearance. I'd be hard pressed to say yes to that request unless it was in a custom campaign setting and not the realms, or maybe if it were a literal reskin where we were using the stats for the race but the fluff was some other kind of elf, not the usual Underdark etc. lore.
As an aside, in terms of the 'realism' of the fantasy demihuman (or human!) melting pot, keep in mind that real world inheritance patterns, geographic distribution, and so on do not have to apply in any way to your campaign setting. You can have skin tones/general features for elves be inherited entirely from the mother, rather than mixing between parents, so half siblings can look as different as Japanese and pure sub-Saharan African descent. You can say that the forest centaurs have always looked more Mediterranean, with dark hair and olive skin, but that those born during the full moon have blonde hair, bronze skin, and golden eyes. You can have your dwarves all have essentially the same core skin color, but those that live deep down in the mountains and never see the sun have pale Caucasian appearing skin, while the more time they spend in the sun the more they gradually tan until if the live like the surface folk they appear completely black.
I mean you could also just say "sure Stan, you can play a white Aasimar, just know that most in this setting look visibly different from you and you'll be unusual." You don't need a big in-world explanation for it unless your players are interested in that, you plan to make it a plot point some day, or you are just a world building junkie, all of which are totally legit but probably aren't important to most(?) games.