Zendaya has developed some serious acting chops with her roles in the Disney Channel series Shake It Up! and K.C. Undercover. Her skills and beauty will blend very well with the current cast of Spider-man and make for a very interesting current installment of the Spider-man movie series coming out soon.
Not only does Zendaya have the benefit of the experience of all the directors who made Spider Man films before him, he also seems to have a better script and a better grasp on what makes Spider Man so unique and appealing. The other Spider Man films were great, but this has more potential.
I do not feel at this point that any addition to the cast can improve the prospects of Spider-Man: Homecoming. The franchise has been restarted and rebooted so many times now that any sense of originality or character held by the IP has drained away and a new actress won't help.
Zendaya has not been chosen beaca use of her acting skills but by the fact she is a product to be placed. She is a popular symbol of american teenage dream, she is a singer too. Being popular is not a good reason for embracing any cinematic roles. You have to be skilled for the job you've been owed.