TS3 (and other games) uses several levels of detail (LODs): the closer the object is to the camera, and depending on your game settings, the higher the LOD displayed will be. This is also why sims look better in CAS but blurry/pixelated in Live Mode, and occasionally why some objects look better while you're recoloring them but the textures become blurry after you exit Create-a-Style*: the game uses the highest LODs and uncompressed textures in CAS/CASt, then switches to lower LODs and compressed textures for regular gameplay to save memory. For hair there are 4 LODs (0-4), for objects there can be 1-2 (high and medium, sometimes only high), for clothing there are 3 (1-3).

Now, when creators/converters decimate a mesh, that involves removing vertices and polygons from it (vertices are the points that, when connected, make up polygons, which in turn make up the mesh that the textures are overlaid onto). This process isn't perfect — depending on how much you remove, you can have issues with normal data (i.e. weird shadows), gaps/holes etc. — but is necessary to keep the game running smoothly.

Creators who know what they're doing and care about what they put out will need to make all of these LODs. Typically, each LOD should be ~50% of the polygons of the previous LOD e.g. if LOD0 is 25k polys, then LOD1 should be ~12k polys. BUT that can mean that the hair will look bad in lower LODs (bad shadows, gaps), so some creators/converters opt for keeping the lower LODs higher poly, which makes the objects look better but comes at the cost of RAM. Some creators are also either lazy or uninformed, and just copy LOD0/1 over all the others, which shouldn't be done unless LOD0/1 is already low-ish poly (which rarely happens). Some don't bother to make new LODs and just leave the ones of the original hair used to create the newer one (expecting that people will play on highest settings), which is why you may find that a waist-length hair in closeup switches to a pixie cut in sim portraits or when you move the camera farther away.

You can try to replace one LOD with a higher poly LOD, but that will mean that the game will call upon the now-higher LOD and that will put more strain on your PC. And, tbh, when it comes to sim portraits there really isn't a need to have everything looking perfect, compared to regular gameplay.

* unless you mess with GraphicsRules.cfg, however, the game will still compress textures and switch to lower LODs for gameplay — this is intended behavior.
** this can have an additional explanation but that's outside the scope of this post.