Changing default \citet font in biblatex

I guess you're also using \usepackage[french]{babel}? The French module in biblatex (french.lbx) formats last names in small capitals by providing this "local" definition of the \mkbibnamelast command:

\protected\def\mkbibnamelast#1{\textsc{\textnohyphenation{#1}}}

To restore this command to its "global" definition add the following to your preamble:

\DefineBibliographyExtras{french}{\restorecommand\mkbibnamelast}

In biblatex.def the global definition is:

\newcommand*{\mkbibnamelast}[1]{#1}

You can redefine this via \renewcommand*. The code in \DefineBibliographyExtras is hooked into citation commands and bibliography drivers. So the French definition will simply be restored to the "current" global definition.

Update (with babel-french v. 3.2a 2016-03-18)

The command \DefineBibliographyExtras{french}{\restorecommand\mkbibnamelast} no longer works.

To disable small caps in names in french language, add in the preamble the following command:

\DefineBibliographyExtras{french}{\restorecommand\mkbibnamefamily}