In case you haven't noticed the similarity, people like to comment, sometimes in an unkind teasing manner on nearly anything you do to "better" yourself if it's outside typical, and sometimes even then.
If you eat healthy, particularly if it's a change from how you used to eat, you will likely hear some snark. I have even heard snark that I think they meant to sort of compliment me on that? Something like, "Well it's about time that you started looking out for your health".
If you start to do a new healthy routine, like a cardio workout, yoga, etc, you can get people snarking about it oddly. I have had comments like, "Oh you don't need to do that. You don't have any weight to lose". I wasn't doing it for weight loss, but anyway...
I even had people be rude about me going to college. I know that sounds really out there to many, but I don't come from an area where people often go, at least not when I was that age, and in my extended family it was a mixed reaction. Some were great, but I also heard a lot of, "So you think you are too good to (insert whatever labor job here)?", also things like, "Who do you think you are with your nose in those books like you are better than everyone else". Even my father made rude comments, but he was/is an under educated man who is defensive on that issue. He chose to see a pursuit of higher education as a statement that his level of education wasn't "good enough".
So yes, I know people will snark about self help books. They also will about therapy, or other ways you may choose to attempt to tackle internal struggles. All you can do is deflect, and remind yourself that what other people say is about themselves. It's totally true. I worked with the public for many years, almost 2 decades, and then did business to business. What I can tell you is that kind people act kindly. You can mess up their day with a mistake, and they are generally going to gracious, understanding, and cooperative while you work on a resolution. Rude people are going to be rude. You can bend over backward for them and very often you will be treated not much better for it. There are of course exceptions, when kind people are having a lot of stress and an off day, you may see that, but again, it's still not about you. I could make the same error with 20 people and I would get 20 different reactions, even if what I did was identical and the way I tried to fix it was identical.
When snark bothers me, I take several deep breaths and say to myself, "I am sorry they have this type of issue that they need to try to make me feel bad in order to feel okay about themselves. I am glad it's their issue, and not mine. I am glad I don't have to internalize that or carry that weight with me," and then I absolutely let it go.
If it were one person over & over, then yes, there are things you can say and ways to try to establish better boundaries. As a general topic to deal with it as it comes up though from an assortment of people, you deal with that internally with how you choose to feel about what has been said. Choose to feel bad for them. Something is clearly bothering them if they feel the need to poke others for no good reason. So if anyone ask why you are reading anything, you can simply say, "Because it interests me" and let the rest go. They will back off already if they see it doesn't matter to you at all what they think of it.