As near as I can tell Dungeon Defenders doesn't have a stand-alone Terms of Service document or EULA: it's not mentioned at all in the standard places in the Mac App Store (or Steam for that matter), and looking about online, it seems any time anyone asks about such a document on the TrendyNet forums, they get the run-around:
In "EULA or TOS", a user asked where such a document was for the purposes of determining if it was okay to make YouTube videos of the game, and was just told it was okay by the administrator Pmasher with no link to a document for proof.
In "Trendy re copyright", a user again asked for proof it's okay to make YouTube videos of the game, and rather than provide a legal document, another administrator, IceArrow, added a note to a FAQ on the forum.
Elsewhere on the forums, people mention that Trendy never responds to requests for an EULA or ToS document, and that no such EULA exists (or at least can be found) when the game starts.
The closest I found to something that describes what you can get banned for is from the aforementioned forum FAQ:
What can get me banned from Ranked?
- Selling items, mana, power leveling services, or map runs for USD or actual Team Fortress 2 Game items(the TF2 pets/portal gun are okay).
- Selling or using hacked items.
- Using 3rd party programs to modify your account.
If you do receive a TrendyNet - Ranked ban and it doesn't get lifted you will still be able to play on Open.
So while botting is technically not against the Terms of Service (because it doesn't exist), and subsequently there are no consequences arising from violating the ToS, Trendy takes a Miller-esque "know it when I see it" approach to bans and things they find problematic. Per the forum post, using "3rd-party programs to modify your account" (botting would count) is something they've decided is a bannable offense.
While you're using the Mac App Store which does not have similar countermeasures, for the sake of completeness, Dungeon Defenders for Steam does have Valve Anti-Cheat (VAC) enabled, so you're subject to the provisions of that program when playing it through there:
Any third-party modification to a game designed to give one player an advantage over another is classified as a cheat or hack and will trigger a VAC ban.