It looks to me like the OP has constructed a straw fattie.
The Body Acceptance Movement is about not hating yourself and your body no matter what the size and to try to be happy and healthy on your premises. To stop obsessing and live your life now instead of "later when I'm X lbs smaller".
As evidenced by my own petite self, people of every size are welcome to accept themselves as they are. It is not a requirement to be above any arbitrary and disproven measurement to feel welcome at body acceptance blogs or forums. So to answer that question, slim people are very welcome. Although any attempt at shaming or proselytizing will go down poorly.
As pointed out by SezMe the generally accepted Stanford study that checked morbidity showed that the only categories that had any increased morbidity was underweight (by post 1998 bmi charts) and the morbidly obese (by same chart). The underweight were slightly worse off, then the morbidly obese followed by the normal weight group. The "somewhat overweight" and the "overweight" groups had the best survival but generally the differences were pretty small and it is hard to say that any particular weight in the span that was neither underweight nor morbidly obese is healthier. Please note that this study used post 1998 bmi cutoffs as measurement. BMI is of itself only useful on population level and the decision to move overweight from pre 1998 bmi 26 to post 1998 bmi 24.9 made millions overweight over night was made for no really good reason. - When people start going on about health around fat people they almost never bother to actually check their facts.
I never take fat haters seriously when they start going on about health. They are really only looking for an acceptable target to pour their hatred and their own body issues on.
So the next time you feel like bashing a fattie and congratulating yourself on your only achievement in life being remaining a certain size - ask yourself this: if there was enough evidence that the healthiest weight category is that one or two pips above mine - would I then go out of my way to gain and retain the "missing" fat for the sake of my health, despite what problems that would cause in my daily life and how hard I would have to work to maintain that? (If you have a quick metabolism for example, you might have to plan your entire day around eating so you don't miss important calories. You would also have to keep eating even after you are stuffed, to keep this up.)
Well, would you?