In Sichuanese does 日本人 mean “fuck me”?
I am a native Sichuanese and have been living in Sichuan for decades.
I personally have never heard of anyone using this phrase and don't think I ever will.
Instead of a everyday phrase, this is more like a joke playing a pun on the character “日”(used both as a noun and a verb in Sichuanese) that you may find on some sites online. And I don't recommend using it in your daily conversations with anyone, because a)it sounds extremely awkward to me and b)it's very rude (and racist) to talk like this.
As for the slangy use of “日” as an exclamation, yeah, you can hear it here like every day. An equivalent in English would be "shit" or the F word. But still, it sounds rude. I would not recommend this usage to people you are not familiar with.