Hi there,
I am assuming you're on a windows system. The best way to solve this problem is to change the graphic device you're using. I have found that cairo works really well for this.
See example below of how to force cairo as your graphics device
ggplot(iris, aes(x = Petal.Width, y = Sepal.Length)) +
geom_point() +
stat_smooth(method = "lm", col = "red") +
labs(title = "Normal") +
ggsave("normal.png") +
labs(title = "Cairo") +
ggsave("cairo.png", type = "cairo")
Here you can see an example too: "cairo" as default in ggsave & RStudio Plots pane in Windows?