Finding next greater element
Start by setting your res list with \$ 0 \$ values. Thus:

res = [0] * len(temperatures)


You won't need the elif condition now. Nor would you need the ending res.append. All res.append inside the loop will be replaced by res[i] = counter values.

Use a more descriptive variable naming than skipped vowels (counter instead of cnt, result instead of res etc.)

You compute len(temperatures) a total of \$ n^2 \$ number of times. Keep the value inside another variable instead.

As for the complexity, your current algorithm is \$ O(n^2) \$, which might be reduced by using a more efficient algorithm. Take a look at this SO answer making use of a LIFO queue (stack) and getting the results in \$ O(n) \$.