Does welding aluminum weaken it?

No, but a lot of people weld aluminum incorrectly.

Enter the TIG welder. This uses a heated tool called a wand rather than a torch (though it is still called a torch in some areas) with your rod as a separate tool.

The problem with aluminum is that if welded by conventional means it forms gases within the weld literally foaming inside the molten metal. This creates a weakened weld.

So, if done correctly using the correct tools, no. If you try to weld it like steel or other materials, yes you are very likely to weaken the material.