She is not a Marans either because Marans have feathered legs, while your chicken does not. That means she must be a mix between different breeds. I am not sure what, but Cuckoo Marans is likely in the mix.
Not every Marans has feathered legs.
Even if you want to say that a "good" Marans will have feathered legs, many chickens belong to a breed without being good examples of that breed.
If she grows up to lays dark brown eggs, she probably is a Marans.
If she grows up to lay blue eggs or green eggs, she is definitely an Easter Egger.
If she grows up to lay dark green eggs, she is an Olive Egger (still an Easter Egger, just one that lays a specific shade of green instead of anything in the blue/green spectrum).
Leg color is definitely a crucial factor in determining breed. Some breeds look similar and can be difficult for a beginner to identify apart (but are easy if you care about knowing breeds), but their leg color will easily tell what they are. For example, inexperienced chicken-keepers could mistake an Orpington for a Cochin. But Orpingtons have white legs and Cochins have yellow legs (and also there is the very obvious fact that Cochins are feather-legged while Orpingtons are not.)
Yes, leg color is an important detail.
But unfortunately it is not rare for hatcheries to send out chicks that have the wrong leg color. I've had Old English Game Bantams that had white legs and ones that had yellow legs (same shipment, same color variety, but only the white legs were correct for that color variety of that breed.) I've had a Bielefelder with green legs (should be yellow.) Plenty of other people have similar stories.
This happens with other traits too. Examples include breeds that should have a rose comb but some individuals have a single comb, and breeds that should have muff/beard on the face but some individuals lack it.
So I would definitely consider leg color, but keep in mind the chance of an individual chicken having it wrong.
In this case, a clean-legged Marans (no feathers on the feet) is more likely than a white-legged Barred Rock, so I think the chick is probably a Marans (unless she lays blue or green eggs, which would 100% point to Easter Egger.)