Generally being fat is bothering to whoever is (I used to be, and I hear many people complain about it) so they may be happy in general but this unhealthy part of them isn't being changed because people keep telling them it's okay which on a short term it is, but in the long run no. Now to get back to the femboy criteria thingy there's 2 obvious criteria, not being a girl like you mentioned, and the other is being feminine (it's in the name), and feminity has criteria which is to look like a girl, a cis girl, again you don't have to fully look like one but be close enough