I like what Helena said. I think the term can be misleading or open to a LOT of interpretation.
I don't think of the term "effortless" when I choose an outfit or style an item. I think of "ease" and whether the outfit/item fits my body, mood, occasion, climate, and my personal style preferences. Yeah, it requires a little effort, but not a tremendous amount. If I'm leaving the house, I will likely put on at least a touch of makeup (tinted moisturizer, mascara, lip tint) and make at least a stab at being presentable, even if I'm just running to the grocery store. I don't put on makeup for a pilates class or other workout -- just whyyyy? If it's a bad hair day, I'll pop on a hat. Small effort, but enough to look/feel like myself and not feel embarrassed if I run into someone I know.
I want to look like I put in a little effort but not too much, ya know? I think each one of us has a threshold, a balance where if we veer too far into either an overly low-maintenance or overly high-maintenance look, we feel either slovenly or overdone. My perceived "effortless" might require a lot of effort from someone else, and might be perceived as not trying enough by yet another person.
So...if pressed to use the term "effortless" to describe someone's style, I think I would be most inclined to use it if I see someone whose outfit, styling and grooming seem very in line with their persona and personality, if they seem physically and psychologically comfortable in their clothes, and if everything seems to fit nicely. Tight outfits don't look effortless to me, for example. "Effortless" also seems to imply a certain level of restraint. Overly-accessorized looks or very blingy looks don't seem effortless to me.
Does that all make sense?