I use LaTeX to typeset my resume and my business card. I have typeset it once, and I can go back and modify the content, and as long as I like the overall shape of the document, and I don't have any spelling errors, I know my document is completely free of formatting consistency problems.
Any small change will be immediately applied to all of the appropriate content.
It also allows me to comment out sections and make comments in the source of the document that I use to tell years-older me why I did what I did.
To me, as a LaTeX user, meeting another such user immediately puts them at a higher level than a non-user. To contrast with Barth's answer, with a flat-out, "No." I would say it's a positive signal consistent with the idea that you can make an investment in learning something, even if not directly related to your discipline, with clear long-run payoffs, and ceteris paribus, I'd rather work with you than someone else with otherwise identical attributes.
But I personally value diversity in learning, whereas PhD programs prefer focus.
Overall, it would be a weak signal and I would not do it simply because you think it makes you look good. Rather, do it because of all the payoffs I mentioned above.