Can I make siunitx commands use serif fonts like the rest of the math in beamer?

siunitx checks if the document font is sanserif using the \AtBeginDocument hook, and adjusts the default if it is. That reflects the fact that in most cases if people set their document all sanserif the desired outcome is that siunitx also uses sanserif.

To reverse this, yo need to alter the settings after siunitx has done its test. That's easiest to do using \AtBeginDocument

\usepackage{siunitx}
\AtBeginDocument{\sisetup{math-rm=\mathrm, text-rm=\rmfamily}}