Is 弄 + complement of result only used for actions with a negative outcome?
No，it can be followed by positive outcomes.
In your examples, they are followed by adjectives such as 乱、坏、脏, which are correct.
However, you can also say

我的电脑坏了，是李明帮我弄好的。My computer was broken, Ming Li fixed it for me.
你能帮我弄一下吗？Can you help me to do/fix this?
他把这个弄得很漂亮、很整齐。He made it very pretty and tidy.

In conclusion, 弄 can be followed by both positive or negative outcomes.