It is better to keep your mouth shut and let people think you are a fool than to open it and remove

I think Mark Twain is saying that it’s better to stay quiet than say something garbled or irrelevant. Fruitless input into something is pointless. The idea Twain is presenting is if you don’t have anything smart to say, don’t say anything at all. The satirical aspect shows that people are stupid; if you’re a foolish person, keep your mouth shut. The world will be better off in silence than inundated in stupidity.
I agree with the quote. If you’re in a comprehensive setting, such as school or work, stupidity is uncalled for. Save the foolishness for a more personal setting. Zealousness and sanguine are one thing; there’s nothing with being happy. But being overly giddy to the point where you’re annoying and distracting to others proves the point of the quote.
The quote is still true today. When I walk into a class ready to work and learn, it’s annoying when people decide to be foolish and prevent the rest of us from getting things done. It would be better if they remained phlegmatic than saying too much and getting a stupid reputation.