Math, at the age where Barbie hits her prime demographic, is no harder than reading, history, singing, or being physically fit. There are exceptions where certain things really are hard to some people with disabilities (both mental and physical), but for the vast majority its not hard - it just takes practice and study/work.
Saying "Math is hard" elevates it and offers an excuse as to why you aren't doing well at it. If you don't read, you'll never be a good reader. If you never do physical activity, you'll never be in good cardiovascular shape. If you don't study history, you shouldn't expect to be able to recall historical facts and make logical connections between events. Playing piano will not work out well for you if you never practice. In that sense, all those things are "hard" - but only "hard" as compared to, say, watching a movie or drinking a slurpee.
Misogyny is presenting a girl as an incompetent fool, incapable of doing the very things which the presentation aims to promote. Apparently, writing even the most basic story book an staying true to the subject is hard as well.