Amusingly eccentric. (Oxford Dictionaries)
Done without considering the consequences; foolish or reckless. (Oxford Dictionaries)
Marked by capriciousness, recklessness, or foolishness. (Merriam – Webster)
Used to describe silly and funny behaviour or a plan that is very silly and funny and unlikely to succeed. (Cambridge Dictionary)
A madcap plan or scheme is very foolish and not likely to succeed. (Collins Dictionary)
Audi Sends in the Clowns for This Madcap Ad About How to Avoid Them on the Road.