It's a farce, people!
This film is a farce. A style of comedy rarely seen in these days of 'cringe' humour (The Office..Borat..etc). It's a style of entertainment very much set in the time period the book was published in (1973) For those of you too young to have ever experienced a proper farce, the dictionary defines it as:
"a comedy that aims at entertaining the audience through situations that are highly exaggerated, extravagant, and thus improbable"
That describes Mortdecai to a 'T' and there's nothing wrong with that at all.
So..all these people who love Anchorman, Borat/Bruno, The Hang-Over, Hot Tub Time Machine, and Judd Apatow films in general are unlikely to find this particular style of comedy to their taste.