They don't write good
That one from yesterday was pretty fun, so let's do it again. Here is another Trump meme/Zapp Brannigan meme mashup. As the one before it was, this is an image of Zapp with a quote from Trump.
The humor in this one is that it is incorrect grammar to say that someone doesn't write good - it would be better to say that they don't write well.
So, this joke is an example of someone making a mistake, and mistakes are funny.