Stupid is libellous. Dumb is someone who cannot speak (though it also has a slang meaning of stupid - which would also be libellous). You are better using the word silly which has no legal meaning.
Nonsense. "Stupid" has no more of a "legal meaning" (whatever you imagine the significance of that two-word phrase could be) than "dumb" or "silly".
"Silly" could be just as defamatory as "stupid", or indeed "dumb".
On the other hand, as countless posts here and elsewhere ably demonstrate, there are many people in respect of whom the defence of justification would be established without a great deal of effort.