Did you miss "testtabell" before "[2,1]" here?
In that case, use the ifelse function, as follows.
---
title: "try"
output: pdf_document
---
some text
```{r}
testtabell <- matrix(c(32, 33, 45, 67, 21, 56, 76, 33, 22), ncol=3,byrow = TRUE)
colnames(testtabell) <- c("1990", "1991", "1992")
rownames(testtabell) <- c("Region1", "Region2", "Region3")
testtabell <- as.table(testtabell)
testtabell
```
some text
some text `r ifelse(testtabell[2, 2] - testtabell[2, 1] < testtabell[3, 2] - testtabell[3, 1], "under", "above")` some test
some text
This generates try.pdf (96.4 KB), and most probably that's what you are looking for.