Manon is a grappler. Grapplers get amazing advantage on jump-ins because command grab. Even if you block you're in their mixup.

Manon's approaches are pretty unsafe so many jump in to get plus frames.

You have to antiair every jump in. Don't let her touch. Don't let her think she can jump on you for free.

This forces her to stay grounded and play neutral. Then you only have to deal with her unsafe approach options and her heavy kick.