Hi,
The template variables are actually output from JavaScript. So it’s not possible to use things like {{poi.title}} in PHP functions.
However, it is possible to trim spaces from the title using the JavaScript trim() function, like this:
<div class='mapp-title'><# print(poi.title.trim()); #></div>
There is some more information in the MapPress documentation, or this page explains more about JavaScript templates in general:
https://codex.wordpress.org/Javascript_Reference/wp.template