You can change the text properties of the plot title with the theme function. See: Theme and Theme elements
Compare these two plots:
library(ggplot2)
Df <- data.frame(X = 1:4, Y = 1:4)
ggplot(Df, aes(X, Y)) + geom_point() +
labs(title = "My Title")

ggplot(Df, aes(X, Y)) + geom_point() +
labs(title = "My Title") +
theme(plot.title = element_text(face = "italic"))

Created on 2021-02-08 by the reprex package (v0.3.0)