I think that the colour setting or filter or whatever used in Stranger Things leeches the colour out of the scenes to give it a more 70s vibe, which may contribute to her skin looking a bit dull and other things people have said on here. Also she is definitely, definitely pear shaped - her upper body may verge on looking almost sickly but her legs aren't the same. Assuming an eating disorder isn't fair.
Yes this - they were supposed to look unwell. ST used a greenish filter to set the era and the eerie mood, and it definitely impacted the cast's general appearance (Winona looked pretty ill too, which was of course appropriate for her character). Natalia looks great in interviews, IMO.