I think it's natural to judge (internally) various situations and things: people/behaviour/appearance/points of view/art/music etc, and those initial first reactions can often shift when more information is processed.
I guess what someone does with those first impressions will determine whether they are behaving appropriately or not. For example, you might meet someone for the first time with awful breath. You could scrunch your nose up and loudly exclaim "ewe" while jumping back 2 steps. Or you could smile and nod and try to hold your breath, and quickly (subtly) escape to make conversation with someone who has better dental hygiene.
You could look at a photo of a woman and think: wow she's ugly. That's a natural reaction, and doesn't necessarily mean that she IS ugly - it also doesn't mean you are unkind. we all have different ways of interpreting and processing the information around us and often it's a reflex that we can't control. If we then go on to post on that woman's photo "you're ugly!" Then that would be cruel.
Since beauty is in the eye of the beholder, I think it's moot what the physical appearance of the person being critiqued actually is. I.e they could be stick thin or morbidly obese, or anywhere in between, and two different people looking at that person might feel completely opposite things about whether that person is attractive or not. But I don't see any reason why someone can't critique another person's appearance if they do it with respect.
Especially when that person posts selfies of themselves online. I'm not referring to anyone specifically, but generally am referring to women who, whether celebrity or not, share their images with the public (for example on Instagram). There is only one reason to post photos of yourself online. To get feedback. That doesn't mean it gives people licence to be cruel. But it shouldn't be surprising when people comment.
I wouldn't randomly go around trolling people's social media but I don't think it's "wrong" to say "I think she is beautiful" or "I think she is unattractive", and if I didn't have something nice to say, I wouldn't say it to that person (or on their Instagram for example) but if I'm watching the bachelor with DH we will happily sit together and talk about what features of which women or men we think are attractive or unattractive. I guess it's all about context, your audience and your intention. If your intention is to hurt someone, then yes it is wrong.
Conversations about men is slightly different because (assuming you are sexually attracted to men generally), it's kind of like rating them as potential sexual partners. This is only natural. Again, I wouldn't condone going on a guy's social media and saying "put a shirt on, your body is disgusting!", but I know I've commented to my friends and online as well about how hot I think certain celebrity men are.. Who hasn't? When it comes to "rating" women, it's not really the same is it..? Maybe that's why there's such a distinction..?