What is going on here is somewhat complex, but there are two main, interacting factors:
The default case in English is the objective case. That explains why it is stupid me, silly me, lucky us. The adjectival use is “recent”.
When something like stupid me pr lucky you is used as a subject, it requires third-person concordance not first- or second-person. This is done for distancing.
Both matter, but the second may dominate.
For the first matter, when we have a pronoun that needs an adjective in English, we use the objective case. This appears to be a recent phenomenon, not attested in Old or Middle English, first appearing in Early Middle English. The first citations of for adjective-qualified pronouns in the OED are clearly in the object slot, but then things get muddy with time:
- A. 1586 Sidney Arcadia ɪɪ. (1590) 179 b, ― Vntil you came, after so many victories to make a conquest of poore me.
- 1608 Shaks. Per. ɪ. iv. 69 ― To··make a conquest of vnhappie mee.
- 1646 Crashaw Poems 149 ― And full of nothing else but empty me.
- 1809 Malkin Gil Blas x. x, ― As for poor little me,··I was sent to the foundling hospital.
- 1814 Jane Austen Let. 2 Mar. (1932) II. 92, ― I am to call upon Miss Spencer: Funny me!
- 1961 P. Dennis’ (title) ― Little me: the intimate memoirs of that great star··Belle Poitrine.
- 1973 D. Halliday Dolly & Starry Bird viii. 111 ― ‘As Timothy would say, silly me,’ Johnson said in a voice as hard as his bifocals.
Another example of this adjective qualification of a pronoun is this time with us, again taken from the OED:
- 1940 M. Dickens Mariana viii. 312 ― ‘How could you know I’d like something like this?’··‘It just looked absolutely us, somehow.’
So when you use an adjective on a pronoun, it takes the objective case no matter what — and it is something of “its own thing”, a set phrase fused together into something new and different structurally, as will be demonstrated in the second matter.
Here first are corpus examples by way of illustration:
- It’s my last table, and the kitchen wants to close. So lucky me has to try and wrangle their order. It ain’t easy. [reference]
- I was just that insignificant little wolf pup, to be kicked around and sat in a corner, because everyone in the world was so right, and only silly me was wrong. [reference]
- Lucky me gets to bide my time with an old man who I saw way too much of during our tinkle break. [reference]
- The dregs of this fair city seem to congregate here every weekend and lucky me gets to stand on the door and watch them bounce in with their put on limps and moody faces. [reference]
- He tried so hard to be nice and most men when they hurt you, they want to have sex afterwards. Stupid me goes ahead in all the pain I was and did to shut him up. [reference]
- So stupid me goes back. I put the fin on the table. [reference]
- Hell if I know why, but lucky me has received six calls now. [reference]
- I like using men as substitutes for my ex-fiancé, since stupid me doesn’t know when to let go and move on with my life. [reference]
- She turned round to face him. ‘But I am, aren’t I? Different. I mean, lucky me has been fortunate enough to be called back . . . to your bed, more. . . . [reference]
Note that this construct preserves number, even though it warps person. So a stupid us still takes a plural verb; it does not switch to a singular one:
- Yet stupid us get baited again and again and again with campaign promises. [reference]
An especially interesting case is this one:
- Just think, lucky you gets himself a whole twenty four hours of looking after this little kitten with no one else around to bother us. [reference]
Again you see it shifting to the third person, but this time coming from the second person instead of the first. Moreover, the reflexive concordance is with himself not with yourself. These are all third-person usages.
So what happens is that you are taking ADJECTIVE PRONOUN, which takes the objective case, and then turning around and using that as a third-person subject, even when it is a first- or a second-person thing. This distances the speaker a bit from what they are saying, as though it were someone else they were talking about. It is hard to see how else one would phrase such a thing. Consider:
- Stupid me gets to work late.
- Being stupid, I get to work late.
- I stupidly get to work late.
Neither of those second two examples works as well as the first one does.