Commenting on other people food diaries?

That's really going to depend on the person. And your tone/approach.

If someone pointed out a very wrong food entry in my diary, sure. Asking questions about the foods I consume, sure.
But commenting (=judging) my sugar intake or some other aspect, yeah no. It's not creepy, but it's not welcome either. My diary is open to show that you can lose weight/be at a healthy weight and not only eat 'health' foods, it's not an invitation to be critiqued 😉

It might be different for people who you know are struggling or still trying to figure things out. For example, if someone complains about being hungry all the time, pointing out low protein intake or lots of sugary/fast carb foods and suggesting things that person can try to not be so hungry, that might be more acceptable. But no guarantee the person on the receiving end will appreciate the remark.
Whereas "OMG, I can't believe how much sugar you're eating, that's awful", that's just a bad idea all around 😉

Obviously, it's going to matter a lot as well whether or not you know these people whose diaries you're commenting on. I wouldn't recommend randomly commenting on diaries of people you don't know, it's a bit different if you're friends with people.

Maybe you shouldn't be reading other people's diaries, if it's for anything other than supporting your own journey.

Even a trained nutritionist needs to be invited, and paid for their advice, rather than providing it unsolicited to Becky at the BBQ with the sausage in her hand.

There is more than one way to skin a cat, you'll see this as you move through your journey. The biggest thing you need to ask yourself is, are you supporting someone by commenting, or are you potentially setting their journey backwards.

Maybe that diary your reading was not even being tracked 3 months earlier, and these are the first steps the person is taking to change their lifestyle, or maybe this is just the way they choose to eat.

My diary vs my knowledge base are worlds apart right now, which is probably important to consider before offering unsolicited advice, maybe that person you're about to provide advice to, has a wealth of knowledge, far exceeding your own knowledge base, maybe they're still putting the pieces together to get back on track, and you pulling apart their choices is not what they need, I kinda think it actually goes against the Myfitnesspal community also.

Big solid, No, for unsolicited advice.

Personally, I'm fine with people reading and asking questions about my diary. I'm a distinctly sub-par MFP friend, but on some threads where people were asking about vegetarian eating, when it was appropriate, I'd tell them to friend me if they wanted to see what I eat to get protein or the like. (I've been veg for 47+ years.)

If someone criticizes what I eat, it will probably make me think not well of them, mostly (unless I think they're right!) . . . but it won't hurt my ego. Sadly, my ego is much stronger than being wounded by comments from random online strangers.

If/when you comment, it matters what you say and how you say it, IMO.

I'm completely open to "how did you prepare XYZ veggie" or "did you like those tofu noodles" or "what does that brand of ABC taste like" or whatever.

Want to tell me I shouldn't have the occasional over-goal-calorie day, drink some wine sometimes, eat something deep fried, be willing to eat 200G+ carbs daily? I'm going to be amused, maybe respond sarcastically. I try not to be truly mean to ignorant people, though, honestly. Sometimes I succeed.

I dunno. Try it and let us know how it goes.

I closed my diary partly for this reason. I find it very intrusive if people start telling me what to do UNLESS I ask them first.