No. Not at all. Being sedentary and overweight is obviously not a healthy thing (*taking into account the few people who have, for example, higher BMIs due to being extremely fit athletes, and also recognising that not all overweight people have health problems, at least not initially!)
It is not ideal to be a 'fat lazy slob' or to have a 'terrible diet', of course. And promoting healthy lifestyles for health reasons (physical and mental) is always a good thing. Looking good is also a good motivation for a lot of people. Some people will particularly enjoy fitness, sport, exercise, have an interest in the scientific and nutritional side of things etc etc, and that is fine. Others will not have that interest, and that is also fine. Not being lean/muscular doesn't associate with a lack of self-control, no ambition, being unattractive etc - there are a lot of other ways to have those traits, whether that's through work/career, art, music, fashion, writing, creating things, whatever. Some people better themselves through volunteering, fundraising, setting themselves a challenge, etc. For some people there may be those associations, but it is definitely not a certainty.
'Bettering yourself' does not mean 'becoming athletic/hot not just getting into reasonable shape' and it does not in any way make somebody 'happy being an unattractive loser'. For the average person (by average, talking about someone who is not particularly interested in fitness, does some exercise but not loads, eats an 'average' diet), junk food and being sedentary at times will be fine. As long as it is not all the time. If on the whole a person is eating a reasonable diet, lots of protein, fruit, vegetables, etc etc, and meeting at least the recommended physical activity guidelines, then having junk food sometimes or having a day of being sedentary, lying around for a bit is not going to cause massive issues. (Just as an example, since inpatient treatment for anorexia, we have been encouraged by the dietitian to have chocolate as a night snack. For me, that is part of a 'healthy' diet now. I am not a 'fat, lazy slob' and I am in reasonable shape with fitness (although still building up strength following anorexia!), certainly without a terrible diet (ignoring the whole, anorexia = I used to have far, far, far too few calories...). I have plenty of fruit and vegetables, protein, carbohydrates, and I also have chocolate sometimes (in fact, I have just brought it back into my diet because I was becoming afraid of it again, and that is absolutely not healthy).
But basically, the average person has no issues at all with having occasional 'junk food'. Eating cake or junk food not does = failing to improve yourself, a lack of self-control and physical ambition. In fact, taken the opposite way, eating junk food and knowing when to stop, when you are full, when is 'enough' junk and how to balance that into a balanced diet arguably shows just as much self-control. Associating junk food with fear and anxiety is not a 'controlled', healthy thing to do. Associating certain foods with fear and anxiety is an eating disorder. Not everyone likes to eat those foods, and for fitness/sport/training/whatever reasons people may choose not to eat them. But being afraid of them is, in my opinion, an eating disorder.
/TL;DR: No. Being sedentary or overweight is not great, including 'junk food' can be part of a healthy, balanced diet, and being afraid of cake/junk food/not exercising enough/etc is not healthy.
Edit: Oh dear God. I've written an essay. If I spent as long writing my stats assignment as I did writing this, I'd be sorted by now...