The formula is
$F = 9/5C + 32$
$F$ increases as $C$ increases and $F$ decreases as $C$ decreases, but the relation is not a direct proportionality.
In mathematics, a direct proportion has the form
$y = mx$,
where $m$ is a constant.
If the formula had been
$F = 9/5C$
it would have been a direct proportion.
However, the addition of the $+32$ term makes the formula no longer a direct proportion.