I say yes, but it doesn’t mean all middle children fit the characteristics. Gender, spacing between children, situation of the parents at the time of birth and years after, even situation of the country you live in can affect each child.
I’m the oldest of two, so no middle child in my family. My parents were the oldest of two. Just my maternal grandmother was a middle child, she was the only girl of three children, but her dad died when she was five and that was such an unusual situation I wouldn’t even count her family in any sort of antidotal evidence or theory.
My husband is the baby of three, and his brother is the middle child and definitely had more struggles, but he’s gay also, which played into some of it. His dad was particularly harsh on my husband’s brother. I think by the time my husband was born his parents had relaxed a little.