Given that currently natural=spring is rendered as a blue circle, I don’t see why rendering natural=hot_spring will clutter the map. Hot spring is clearly just a subcategory of spring. The blue circle can be slightly modified, adding two or three S-shaped lines above it to represent the vapor, to depict hot springs, or if the developers are too lazy then simply reuse the blue circle should be totally fine.

These are just nodes, not areas so they are not taking that much space to the point that the map becomes cluttered.