For images to MIPMAP, they MUST be a POW2 size on X and Y (power of 2; 1, 2, 4,8,16,32 etc.). Images on GameMaker aren't. Now... I can automatically scale everything to POW2 (the C++ runner already does), but making everything a MIPMAP also has issues. 1st, you don't want SPRITES to "mipmap" when they scale, they would just blur and go horrible if you shrink a sprite.
This means you really need an option in GameMakers sprite dialogs to say you want something mipmapped, and then we'd have to enforce POW2 constraints. This is okay... but it means it's a bit of an effort to setup.
The other thing we could do is to give you a call in code to enable MIPMAPPING in GML, something like this...
We're still discussing the best way of doing this, but it's clear we can't just "switch on" mipmapping, there are too many issues for other types of games.