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Nick Diaz says Michael Bisping and Georges St-Pierre are STUPID for not fighting him -- telling TMZ Sports he'd make them both a lot of money ... but they're too chicken to take the match.

We got Diaz leaving the UFC 209 in Vegas ... and after he was done insulting UFC welterweight champ Tyron Woodley ... we asked him his thoughts on Bisping & St-Pierre locking horns in the future.