1.) Open a text editor.
2.) Paste the following lines;
mat_filtertextures 0
mat_filterlightmaps 0
mat_mipmapping 0
mat_antialias 0
-and save the file as autoexec.cfg.
3.) Place this file into C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\cfg\
4.) Reboot the game
-You can also sample these commands ingame after enabling the console in Advanced Settings and then pressing ` and writing those commands.

Other useful things for this game are;
Turning off cosmetics - I think it looks much better, check it out. https://gamebanana.com/skins/download/169236. Throw this mod into your Team Fortress 2/tf/custom folder. It works in Valve servers.
Turning off weapon skins - To do this you have to switch to Directx8, which I think is only an option for Windows users.
1.) right click on Team Fortress 2 in Steam
2.) click Properties
3.) then click Set launch options
4.) paste -dxlevel 81
5.) run Team Fortress 2 once, get to title screen and then exit the game once more
6.) finally go back to the Team Fortress 2 launch options, remove the line -dxlevel 81, and then just open back up the game to play. This is specifically because for whatever reason while this is kept written you can get blackscreens whereas simply removing it fixes these
Changing default HUD - You have a lot of options but you could just google for that. I use ToonHUD which can be rather nice with configuration.
Enabling high texture quality - By default "max" in settings is not max. Typing mat_picmip -3 will enable the highest quality textures. It is very apparent on miscellaneous items around the map like spray cans. EDIT; this is now disabled
Disabling shiny textures in directx9 - This command will make textures look more like the material though less cartoony. mat_phong 0
Disabling ads and shit on custom servers - However this is a tradeoff, while adridden bullshit wont be viewable you also wont be able to for example type !logs in an organized lobby and will instead need to view the stats in your webbrowser. cl_disablehtmlmotd 1