I think it's not about body mass ratio or fat percentage, so much as about health more generally – people who take care of themselves show a capacity to "take care" of others, in both the "Florence Nightingale" and "Florian Nightingal's All Nude Revue" senses.
Still, that leaves a huge swath of body types, height/weight combos, etc that are capable of "finding(*) true love"… The way to improve one's chances at love is to work solely on the aspects one can control. If one of those things is your weight, BMI, waistline, etc, go for it – if you do nothing other than show "true love" for yourself, then you will still have found it, no?
(* More on the finding issue here: Where and how can I find true love?)
Do thinner people have a better chance of finding true love?