It really depends on how you present yourself and how the self promotion looks itself.
In my opinion you should do that at the beginning until you get a small portion of subscribers with whom you can work with further on. Like what are the odds that someone, for example, will type my username in Youtube search bar - none, no one even knows who I am except those few who subscribe. And self promotion in big youtuber comment section is one way of getting that start-up subscriber amount. Of course, you can do that through your friends, family, forums and what now. But, in my opinion, youtube comment section is one of those ways. Of course, it gets annoying when every kid writes "Yo, check out my cool minecraft vids", but there are some that explain the situation and are very polite on this matter. Imagine how faster things can go if you get like 300 subs at the beginning and interact with them. Of course there are these "hard beginning times" but you can always go past that by just few steps.
Me, myself - I am a new youtuber and at the beginning I had no subscribers. I shared my channel url with all my friends and that gave me about 35 subscribers. Then I did self promotion on one big youtuber comment section. But it was not one of those cheesy self promotions. Here's how mine looked "Hello, guys. I am sorry to bother you, I did enjoy video but I am also here to ask you a small favor. If you could spare just 5 minutes of your time and check out my youtube channel, you may find some good quality videos that might interest you. I know this is not the best way to do it, but all I am asking you is to check it out, maybe you will like it. And of course you can just ignore my comment and move on." It's just a short example of what I've done. And I didn't do that on like every video I watch. No - I just watched one funny video of some youtuber and posted that sort of a comment. As the time went by I kept on continuing the topic and replying to people who considered that as something wrong.
At the end I earned about 80 subs and for the beginner that is a lot. I am happy for what I've got and this is my opinion on self promotion in big Youtuber comment sections.