It can be. Sitting still is understimulating, and we tend to not like that. However, there is also research that shows we have poorer posture and vestibular balance, which can also make sitting harder (like most ADHD research, it’s done with kids):

Children with ADHD often demonstrate poor posture and postural-ocular integration which can be manifested in trouble sitting still, taking frequent movement breaks, or slumping/leaning. Emerging evidence suggests that vestibular brainstem reflexes are altered in a subset of children with ADHD and points to this as a cause of decreased postural control.

Source: Isaac, V., Olmedo D., Aboitz F. and Delano P. (2017). Altered Cervical Vestibular-Evoked Myogenic Potential in Children with Attention Deficit and Hyperactivity Disorder.