The Thumbor servers currently use fonts-liberation 1.07.4 for the Liberation font family. Liberation Serif is used as the default font for SVG rendering , but Liberation 1 doesn't always produce nice results at smaller sizes (T247567). I would suggest moving to fonts-liberation2 for thumbnail rendering. It is more actively maintained, and produces generally better results.
Test files are https://commons.wikimedia.org/wiki/File:Sea_level_history_and_projections.svg at 270px.
Liberation Sans 1
Liberation Sans 2