don't be a wally

Don't make yourself seem, or act like you are less intelligent than you actually are.
The actions of someone appearing to be like a Wally, may be either unintentional or on purpose.
Being likened to a Wally, can happen either affectionately, in a sarcastic and demeaning way, or as encouragement not to be stupid.

"Hey mate, don't be a Wally."
"Ha-ha.... Don't be a Wally."
"That's amazing, but don't be a Wally."