A blog post on SimCity's official site encourages players to create and download mods for the game, but asks that they do not "jeopardize the integrity of the gameplay or harm the experience of others."
The post by the SimCity development team says that Maxis and Electronic Arts have a long tradition of supporting the modding community, which they wish to extend to SimCity as well, but only if they follow a list of rules.
The rules are largely predictable, asking modders not to add copyrighted or adult material into the game, and prohibiting them from selling the content they create. EA reserves the right to stop players from distributing any mods that violate these rules.
More vague is the rule against mods that impact the simulation of multiplayer games and features, such as leaderboards or trading with other players. SimCity players have been vocal about their issues with some basic design decisions in the game, and would presumably enjoy mods that change them. The rules don't make it clear that such mods would be allowed.
Also limiting modders is that SimCity still doesn't have an offline mode, though a recent post from the development team on EA's official forums says that it's still exploring one, as it "will give our players more room to experiment."
In March 2013, EA said that an offline mode didn't fit with its vision for the game. In October 2013, after a strong response from the player community, Maxis said it was looking into adding an offline mode.
At the Game Developers Conference 2012, SimCity creative director Ocean Quigley (who has since left EA) said that the he and his team are huge fans of the modding community and that they've "designed all this stuff to be moddable."
You can find SimCity's official modding policy here.