The computer never mulligans. Truthfully I think I could program the computer to mulligan but I didn't think that it would greatly improve the AI.
When a player is deciding whether he should take a mulligan or not many factors flow through his head such as "Do I have the lands that I need to support this hand?", "Is this deck slow or fast?" and "Can I win the game with this hand?". I can't code any of that stuff so the computer would have to follow a few basic rules such as to mulligan if he had only one land.
MTG Forge has an option called "Stack AI Land" which can be turned on or off. When this option is on it stacks the AI's library so that he will draw a few lands to begin with and the rest of the lands are put on the bottom of the deck.
Theoretically this should give the AI a boost in longer games since you would occasionally draw a land while it will be drawing nothing but spells. The "Stack AI Land" option presumes a two color deck, so if the AI is playing a 3, 4, or 5 color deck you should turn this option off.