There is nobody bigger or better at the military than I am
Here is another Trump meme - this one is just an image of Zapp Brannigan, a character from the TV show Futurama and a quote from Trump. it seems that Zapp must be a doofus of some sort, and this is an insult directed at Trump - by making it seem like they are quotes by a comic relief character from a funny tv show, they poke fun at how dumb this quote is.
I am assuming that Zapp is an idiot because it looks like he is - if he is not an idiot than I am not sure why this meme is supposed to be funny.
As a side note, this is probably also calling Trump a liar (because he never served in the military), and this is also an example of hyperbole, but honestly both of those are pretty normal for Trump, and not particularly funny.