If you are using biblatex v3.17 (2022-01-25) or above you can use bibstring formatting sets to typeset "et al." in italics.
Bibstring formatting sets allow us to apply additional formatting to sets of bibstrings. We could, for example, give a set of Latin terms and use italics for those. The bibstring sets are usually defined per language inside \DefineBibliographyExtras, since their logic depends on the actual translation.
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[backend=biber, style=authoryear]{biblatex}
\DefineBibliographyExtras{british}{%
\DeclareBibstringSet{latin}{andothers,ibidem}%
\DeclareBibstringSetFormat{latin}{\mkbibemph{#1}}%
}
\UndefineBibliographyExtras{british}{%
\UndeclareBibstringSet{latin}%
}
\addbibresource{biblatex-examples.bib}
\begin{document}
Lorem \autocite{aksin}
\printbibliography
\end{document}

Some more background and discussions can be found in https://github.com/plk/biblatex/pull/1028, https://github.com/plk/biblatex/pull/1027, https://github.com/plk/biblatex/issues/899.
name:andothersmacro. – aghsmith Jan 13 '12 at 14:04xpatchpackage will make it much easier to make small changes like these to both bibliography macros and drivers. Feel free to accept lockstep's answer instead. – Audrey Jan 13 '12 at 17:52