Hi JuliaMead, and welcome to the forum!

Unfortunately you haven't given the source or context for the phrase you're querying, but it has its provenance in the political mythology of the socialist era, and its impact and therefore true meaning lie not so much in the rather bland wording of the question as in the socio-political context in which it's spoken.
It's really a rhetorical question: the speaker is invoking bragging rights about having an occupation (here, being a miner) that enjoyed a carefully cultivated politico-social kudos because of the nature of the job (dirty and dangerous, but essential to the economy).
So yes, you're right that the words mean "I am a miner, who is more?" or, indeed, "I am a miner, who is better?" but those versions don't really convey the full impact of the utterance.
I think I'd go with "I am a miner, who (among you) can say better?". I haven't
got the context, of course, but I would expect this utterance to occur in a context where the speaker is challenging anyone else to say that they might have a more valuable (in the politico-social sense) place in (a notionally workers') society than the speaker.
Máte ale pravdu, doslovný překlad nemusí být výstižný. You're right, though, that a literal translation doesn't always cut the mustard.
Coincidentally. an editor with Radio Ostrava, the (political) centre of the mining industry in socialist times, says pretty much the same thing below, noting that "
the question was usually followed by silence, partly because no-one wanted to cross swords with the speaker, but also because there was really no answer to this provocative question ...":
So the
emotional impact (though not the translation) is "I'm a miner - nuff said!", "I'm a miner - end of!", "I'm a miner, so don't try it on with me!"