The game's tagline is about "being someone you're not". While this obviously applies to Pizza, it's also an apt description of Chicory, who grows exhausted and breaks down over the course of the game because of trying so hard to be perfect despite her growing anxiety and doubts.
The first place where the corruption spreads is in Supper Woods, where Blackberry lives. Considering how Blackberry's actions in the past played a huge part in Chicory's self-doubts, the corruption growing in the place she lives in is not a coincidence.
Prunus, the Royal Ambassador, admits to Pizza how because of his duties (which involves learning about outside cultures and languages), he's always felt self-conscious about feeling different. Notably among the bugs having a Floral Theme Naming, Prunus refers to the genus that includes many fruit bearing species, so his name fits the Edible Theme Naming of Picnic.
The boss fight against Chicory herself is the only boss where the objective is to not fight back, but to instead wait until she stops attacking. It makes sense for Pizza to not fight back, as doing so would likely reinforce Chicory's view that she is a horrible person who deserves to die. By not attacking, Pizza is showing that they do care for Chicory's well-being, and that she is not someone who deserves the things she suggests to do to herself.
Furthermore, Chicory is the only boss that's an actual person. All other bosses in the game are either results of the Corruption or the Brush itself. Pizza has no intentions of hurting their friend and idol, so they try to Hold the Line until they can get through to her.
