Does more vertices mean better deformation?

A more detailed jbeam skeleton means better deformation. The mesh should be at least 3 times the complexity of the jbeam or alike

 

I've made a quick example



Both planes are deformed using 2 identical boxes like this (Which will act like our JBeam)


The first plane is a simple one. 4 Vertices only.
The second plane is subdivided 4 times.


In the case the mesh has lower poly than the JBeam, the deformed mesh will miss details.
In the case the mesh and the Jbeam have a similar density (or slightly higher), the deformation would look ok.
In the case the mesh is incredibily highpoly, the deformation will start to look funky.

Official cars usually have a poly count of around 60k tris IIRC, which does good with their Jbeam.
If you went on Youtube and saw some videos with those mods like 'Pagani, Lamborgini, etc', they sometimes use models that are much more highpoly, those tends to create 'spikes' when crashing.


So, as suggested above, having a ratio of 1:3 will work fine in most cases.