Boyfriend advice

Is it bad my boyfriend allows his ex to follow him on social media? He posts things of us and me on his Instagram and she likes it which kind of rubs me the wrong way. (She follows him, he does not follow her). I notice whenever he puts something up, then she does this has recently started. Last week, he posted photos of us with his family so she pic up a random picture of her ass (multiple and random guys with inappropriate comments) and I feel like it's screaming for his attention. I'm a very thin girl too and I'm sure she has looked at my page so I feel that it was a shot at me.. She broke up with him 3 years ago, 2 years ago tried to get back with him and he rejected. I just don't understand why my boyfriend will not block her, she is causing fear in me.