Should one build mutual fund investment portfolio based on return only?

Is there anything wrong with the idea of investing in those mutual funds which have given highest return in say last 3/5 years? I do not understand why people worry about selecting diversified equity fund or debt fund; about large cap or mid cap fund. After all what matters is the return, so one should select those funds which have proven track records of better returns than others and select those with maximum returns. Is this idea wrong?