I might be the only one figured this issue, for some reason when I found out the hats wasn't displaying on my view, I thought it was one of my mods or plugins.

It took me half an hour to find out when I removed recording_helpers which shows hidden convars in l4d2, I could finally see the hats in the game.

Edited : okay the main culprit was r_lod which was the hidden convar. If anyone's convar r_lod is 0, revert it back to -1.