You're entitled to your opinion, but I think he just has a quiet personality.
Being in a boy band (or girl band) does not mean he's a professional dancer, and even on Strictly, they haven't always proved to be very good. Examples: Mollie King, Lee from Blue, Max George from The Wanted from the top of my head.
He was in a talent show called Dance, Dance, Dance with his wife, Chloe, but from what I've seen on YouTube, it's just copying the dances of popular music videos.
He's no Danny Mac, but Danny Mac was already exceptional I week 1, and it's not a fair comparison. His Cha Cha Cha was already final standard in week 1, whereas JB's dances were just okay, his jive landed him in a dance off but as the weeks have gone on, he confidence has really transformed him as a dancer.