Being told to smile can be a form of microaggression regardless of intent. Small repeated things that seem harmless enough but chip away at your psychological wellbeing. It's something that many women face, but also a common struggle for autistic and other neurodivergent folks. I fall into both of the above camps and it definitely wears at me. Most of the time I have no idea what my face is doing. Wearing masks has been a great comfort and has helped taken a load off my mind. I'd been wanting to wear masks before COVID but people would give me dirty looks and I was too nervous to. Now I can and I revel in it.
Honestly it all depends. I'm not one to tell someone what to do with their body because it's not any of my business. But a lot of people who are too fat too tall to skinny to short too black too redheaded too WHATEVER get constant microaggressions like mentioned above. Just "smile" "lose weight" "gain weight" etc etc etc. There are Olympic athletes and popular actors who, by all accounts of how the BMI works, would be considered "morbidly obese" but it's all muscle.
Another example: I'm fat, so hearing consistent negging microaggressions about my weight by strangers, family, etc. can wear on me. I don't mind my being fat though it's been something to get used to since it happened somewhat recently. Even at my thinnest, back when I was a model and didn't have enough money for food, people would talk about my weight. Now, in the last few years I became actually fat, unlike how I was when I was younger and curvy.
So if I say, something about being fat frankly, people always go "don't say that, you're not fat, you're beautiful." No, it's an objective fact that I gained 200lbs rapidly due to health issues. (Also this implies that fat and attractive are 100% always mutually exclusive. Which is...not objectively true, considering that what people find attractive is highly subjective.)
Where this gets dangerous is when I seek medical help and it gets in the way of treatment. Not only is this annoying but it's literally endangering me. I had a badly sprained ankle from falling at a train station once and when I sought out help, the doctor told me it'd never heal because of my weight. So I told him to get ready to amputate, since it'd clearly help me lose a few pounds and get rid of the source of my pain. He was rather offended by this suggestion for some reason. :P
Some people have commented that it must be upsetting to have become fat, but they always pin it on aesthetic reasons. Meanwhile I'm more preoccupied with the fact that I have an immune system that is wreaking havoc attacking my own body and I've had to have multiple surgeries and an organ removed. I've got bigger fish to fry than how I look or what size clothes I wear. When I focused on my weight and changed my diet and exercised myself to the point of doing permanent damage, I gained and gained. It was only by tackling the health issues that caused me to balloon up that I started to finally lose weight in the first place. But instead I got to deal with years of "have you tried ___ diet/exercise."
On a slightly different note, something similar has happened with becoming disabled. All the positivity in the world won't change the fact that I have permanent damage and that if I don't get medications I will die. No amount of doing yoga will change this, but random people as well as friends and family constantly recommend it as a cure-all. But if I point out my limitations, people rush to deny them. I point out that I need a wheelchair and people tell me to avoid putting it off as long as possible, as if getting a mobility aid is "giving up." Getting a wheelchair is the difference between me leaving the house at all or being stuck in bed for weeks at a time. The message I get much of the time is that if I can't be healthy I deserve to be a shut-in. It's a message sent loud and clear to most people with a variety of disabilities.
I get that acknowledging disability is taboo in the culture I live in but frankly being disabled is not a question of "if" but "when." Everyone is one car accident, one unexpected illness away from disability in my opinion. Being born, getting older, those are both risk factors as well. I'm weirdly grateful for due to COVID is that it's increased accessibility. I wouldn't have been able to get to doctors appointments this year without being able to do so through the phone/computer.
TL;DR, yes "positivity" can be toxic.