I don't upvote my comments, as I think it would be a waste of my voting power. But maybe I will if I reach dolphinhood...
To be honest, I think Steemit was designed in this way. It's all part of the reward system, to encourage people to invest in Steem and hodl. I've recently watched a couple of interviews with Dan Larimer (co-founder of Steemit), and - I hope I've got this right - it seems that he made a deliberate decision not to adjust the Steemit model as so many people had bought into it the way it was. So I don't see it as "immoral" or "unethical" to upvote your own comment, because the system was designed to make this possible.
If "plankton" are putting their energies into upvoting their own two word comments, I suppose that's their business. I certainly won't be following them or paying any attention to them.