Is there a funny expression for "Keep it simple"?
Solution 1:
I'd normally say KISS ("Keep it simple, stupid!"), where you're advising someone not to overengineer a solution.
In some contexts, "Softly, Softly, Catchee Monkey" may be appropriate. It's usually used to mean Don't alarm {your quarry}, but it wouldn't seem unreasonable to me if you said it to a colleague when you meant We can solve this problem by doing something inconspicuous that won't require us to seek approval from the boss (in the UK, Don't make a song and dance about it.)
Solution 2:
An expression that has similar meaning:
If it ain't broke, don't fix it