I mean... reading is meant to be a hobby you enjoy. Saying people can only read children's books if they read a variety of other genres too is a bit much. Who are we to police what people can enjoy in their own free time?
People can do whatever they like with their time as long as it's not hurting themself or anyone else.
In Ruby's case, she reads exclusively children's books as part of a wider obsession with being a child again. And since that seems to be very clearly feeding into (no pun intended) her dangerous weight loss, ED habits and compulsions that stunt her emotional growth in every way, that's an issue. People are right to be critical of it.
When a grown adult only reads children's books and also sleeps in a child's bed, dresses like a child, acts like a child, starves themself in an effort to look like a child, has their parents chauffeur and chaperone them everywhere to feel more like a child, does everything they can to make-believe that they're a 12 year old at boading school, has child penpals and tapes pictures of children all over their room like a shrine, that's gone far past the limit of being a harmless hobby and deep into the realms of obsessively harmful and intensely creepy.
And then on the other side of it, Ruby reads exclusively children's books but lies about it. She often tells people she reads hundreds of varied grown-up books a year, brags about it how much of a bookworm she is, and allows impressionable young people and children to feel bad about themselves because of it (see the constant "Ruby's reading a dozen books a week and I can’t even finish one because I'm so crap and lazy, I wish I could be like her!" comments).
She's an influencer lying about what she does and who she is to feel intellectually superior to other people when in reality she won't read anything with a recommended reading age above 11. People are right to criticize her for it. She needs to grow up in every conceivable way.