There's probably a lot of ways to do this but here's one way and I borrowed some guidance from here: r - Revisiting the "Format latitude and longitude axis labels in ggplot" - Stack Overflow
library(tidyverse)
world_map <- map_data("world")
base_world_messy <- ggplot(data=world_map, aes(x=long, y=lat, group=group)) +
geom_polygon(colour="black", fill="light green") +
ggtitle("Here's my title")
xr <- seq(-180, 180, 15)
xlabels <- parse(text=str_c(abs(xr), "^o"))
yr <- seq(-90, 90, 15)
ylabels <- parse(text=str_c(abs(yr), "^o"))
base_world_messy +
scale_x_continuous("Longitude", breaks = xr, labels = xlabels) +
scale_y_continuous("Latitude", breaks = yr, labels = ylabels)

Created on 2021-12-20 by the reprex package (v2.0.1)