One option is to use html/markdown in conjunction with the {ggtext} package
library(ggplot2)
library(ggtext)
myvar = "bananas"
ggplot(mtcars, aes(x=wt, y=mpg))+
geom_point() +
ggtitle(paste0("This is ",myvar," and this is my r<sup>2</sup> model"))+
theme(
plot.title = element_markdown()
)

Created on 2022-05-11 by the reprex package (v2.0.1)