I have a good brain
This is another Trump Meme. It is pretty bad - They have an image of Trump making a strange face, and holding his fingers close together as if measuring things, and then saying that his brain is big, but using poor grammar.
This is an insult directed at Trump. The humor is that making him talking about the size of his brain while holding his fingers so close together makes the reader think that he is gesturing to indicate that that is the size of his brain. How close his fingers are together would be a good sized brain for a mouse perhaps, but is not a reasonable size for a human. Additionally, having him say that is brain is big, but use poor grammar to do so is ironic.
Again, this is pretty over-the-top when it comes to insulting him - it feels petty. To me pettiness is not funny.