hmmmm
The Unicode definitions for theta are:
U+03b8 GREEK SMALL LETTER THETA
U+03d1 GREEK THETA SYMBOL
With the first of those being the one in your question, and the second being the more curly open one.
However in the encodings set up by the ucs package a U+03b8 character will by default give you \texttheta (which would probably give you the right character in text mode if you loaded the correct font encodings, I got undefined command) But if you use the [mathletters] option as in your fragment it switches to use the definition
\uc@dclc{952}{mathletters}{\ensuremath{\vartheta}}%
in the file uni-3.def
I.e., assumes that you want the theta-symbol character in math mode. So the \let you suggest would be one way, or you could over-ride this definition:
\documentclass{article}
\usepackage[mathletters]{ucs}
\usepackage[utf8x]{inputenc}
\makeatletter
\uc@dclc{952}{mathletters}{\ensuremath{\theta}}%
\makeatother
\begin{document}
(e.g. θ)
\end{document}
which produces:

The full set of \var... in uni-3.def is
5 matches for "\\var" in buffer: uni-3.def
174:\uc@dclc{949}{mathletters}{\ensuremath{\varepsilon}}%
183:\uc@dclc{952}{mathletters}{\ensuremath{\vartheta}}%
210:\uc@dclc{961}{mathletters}{\ensuremath{\varrho}}%
213:\uc@dclc{962}{mathletters}{\ensuremath{\varsigma}}%
225:\uc@dclc{966}{mathletters}{\ensuremath{\varphi}}%