I kind of skimmed through the replies here and I know a lot of them were written by veteran and more experienced models but I have a different opinion on this issue. The past 8 months I have spent a lot of time observing and learning about the industry, and I know that a few of the "top models" are stick thin, but that has little to do with why they are so successful. There are more top models with a healthy or higher average BMI opposed to underweight. There are tons of studies that have shown that of course appearance is the first thing men notice, and that the majority of men prefer curvier women or women who appear healthy, physically fit. However I don't really think this matters too much as most of us think it would. MFC gets an enormous amount of traffic (check Alexa.com), so not being the girl next door, or the underweight girl next door, is not going to stop you from being successful. Everyone has their "type" or preference for appearance, personality, etc., and even preferences we aren't really aware of. So embrace your appearance and love yourself, you don't need a category slot for yourself.
There's no magic key or guide on how to become a successful cam model. But things I consider to put you at an advantage are; understanding and learning about the psychological aspects of camming, confidence, having an average or above average overall level of intelligence, cleanliness, having a high quality stream, and relentless self promotion via social media. Again these are things that in my opinion HELP.
I started MFC with no experience, no knowledge of the industry, no expectations. My camscore 8 months ago was at 950 or something my first week. My thoughts of myself at the time were that i was too skinny, my arms were too long and lankey, my forehead was massive, I have chicken legs, I have no ass, you can see my scars under my beautiful but fake boobies, I have too many tattoos, I shouldn't talk too much because I am really weird, my boobs are too far apart, and I could go on and on. Over the first few months a few members helped me break out of my shell and I experienced a big change in my self esteem and confidence. My thoughts of myself now are that I'm a freakin hottie and I love myself the way I am. Sounds super corny I know but it's true. And I now have a decent camscore (2400 or something) and make more money than I did as a nurse working 12 hour shifts everyday. Also would like to note that continuing to learn and knowing when to adjust are important in this constantly growing/changing industry. And confidence!