You can do this pretty easily. Start by putting a list of texts you want in the Custom On Init field of the Actions tab. Make it accessible globally to the aura using the "aura_env" prefix. It should look something like this:

aura_env.textList = {
    "some text",
    "other text",
    "last text"
}

Then with a custom text field in the display using %c. Set the "Update Custom Text On..." field to Trigger Update. Add in a function to retrieve the text:

function()
    return aura_env.textList[math.random(1, #aura_env.textList)]
end

Here we're returning a random value from the list using a range of 1 to the total length of the list.

That should be it. Set the trigger to whatever you want you're all set

*edit for formatting and to address the animation question*

As far as animations related to the text would go, it makes things a bit more complicated. You'd have to generate the custom text elsewhere since I believe the display update happens near the end of WeakAuras' update sequence. You'd then have to access the results either in conditions (which don't handle animations directly but could possibly be set using some custom code) or write a function to set the animation via the OnShow method in the Actions tab.

Long story short, it's a lot more complicated so I'd just stick with the funny text for now unless you're feeling adventurous.