What is the simplest way to reduce the roundness of my bulbous nose? Is it just a matter of taking out some fat? Maybe Kybella?

Without pictures or seeing you in consultation, I would be doing you a disservice recommending a specific procedure. If your nasal tip fullness is truly due to fat, using Kybella would be a big mistake! 
There may or maybe not a simple way to have you obtain the result you want other than having you seek a face to face consultation. What you really want is the best result for you and a simple pathway has so many options. During this examination, the doctor can examine your nose and provide you sound advice.  Most of the nasal tip fullness in the cases I performed surgery on was due to the cartilage within one's nose...NOT fat. 
Sincerely, Dr. Kulick San Francisco

Whoa!! Definitely NOT Kybella! The roundnesss is due to the shape of the cartilage not fat. Kybella only reduces fat. Sometimes you can have fillers placed to hide some imperfections but they typically don't work great to make things "smaller". You will probably need a surgical rhinoplasty to make the changes you want. Go check out a good rhinoplasty surgeon in your area. Good luck.