Should I mention a discovery was made by mistake? How is this practice viewed?

The paper is not the place to give a step by step account of everything that happened. You should have a research log, or lab log, or some other form of documentation that does that, but that is separate from the paper. The paper is there to present your finding in a clear and concise way. Based on the paper you should be able to replicate the results, but, depending on the kind of research you do, is not necessarily the same as a complete list of every step taken. I think of the paper as the summary for the research log and code.

I can imagine accidents where it is worthwhile to mention that in the paper, others where it can be entertaining and other accidents where it is not appropriate in the sense that it wastes the readers time. It depends on the accident and the style of paper you are writing, where the latter in a large part depends on the journal you intend to sent it to.

If there was no rational motivation for the discovery, I would write "We serendipitously discovered..."

Some people might just give no reason. Most people (except maybe clickbait writers) will not care if the discovery was an accident.

I think you should briefly describe the fortunate accident that led to the discovery. Those few words will add to the human dimension of science at no cost to the reader and perhaps some benefit: they won't waste time wondering "how in the world did they think to try that?!"

I wonder if Fleming's paper on his discovery of penicillin discussed the accidentally exposed petri dish. A haven't found the paper on the web (after not too long a search).