Jesus is not saying that he'd rather we were cold in our faith or hearts and unbelieving towards him, rather than lukewarm. No Father wishes that from his children.
Think of the attributes of hot and cold.
Heat:
Healing - hot compresses, baths, and heating pads are used to bring healing to sore muscles.
Comfort - hot drinks and sitting by a fire brings comfort on cold winter nights.
Cold:
Healing - ice and cold compresses are used on sprains and sore muscles to bring down sweeling and help heal.
Refreshing - ice cold drinks and dips in cold pools refreshes us on hot days.
Hot and cold both bring healing, while one also refreshes and the other comforts. Lukewarmness does neither. It cannot refresh, comfort or heal. Lukewarmness is good for nothing. Jesus is saying we need to be people who bring healing, comfort and refreshing into other's lives. Hot and cold are both good in His eyes. These are the deeds that He's looking for in His children.