That not possible with Pinterest Theme. But with some customization can be done.
First you need to copy the layout to your wordpress theme. You can do that from Wp Masonry > Layouts. And from the list please Click Copy to WordPress Theme next to Pinterest.
After that, you will get a new location of the layout. Please open layout.php from that new location.
You can simply keep below and it will show the excerpt from your post.
Hope this helps.