Most people involved in fitness, work or hobby, are the least likely to "body-shame" someone who is trying to exercise. There is respect for fellow sport people, whatever their level, and everybody starts somewhere. If there is bitchiness, it's against someone similar (you can have nasty verbal cat fights between 2 zumba teachers for example), it's not against someone who is only starting.
However, you might be mock if:
you turn up caked in make-up, pose, take selfies and even worst chat or rest on equipment so no one else can use them.
If you are on the phone on a treadmill, you do deserve a smirk.
you have an attitude, and think you are an athlete when you are clearly not.
I have seen some obese people (courageously) attempting a very high intensity program with ridiculously fit people. There was never any negativity, they felt embarrassed whilst the others were just thinking "well done to start". Frankly, most people focus on their own exercise and don't have that much time to gossip about others.