What does (MM) mean in the iPad model lineup?
I do not have a direct link or citation on/about Apple to prove my conclusion. But the most logical answer I could gather by reading several sites is that MM would stand for "Multi-mode", signifying that the iPad supports not just CDMA and GSM, but also multiple LTE implementations. The term "multimode" or "multi-mode" is used to designate chips/devices that support multiple, disparate communication technologies (a device with WiFi and Bluetooth can be called multimode too, but we're talking only about cellular here. Plus, all of Apple's iDevices have had WiFi and Bluetooth in all models).


Only the GSM + CDMA + LTE models that support multiple carriers have the "MM" tag.
Example: The 4th generation iPad and the iPad mini have models that support CDMA/GSM/LTE on Verizon and Sprint. So they're marked as "Cellular (MM)" to give them a more substantial (and carrier neutral) name.
The GSM + CDMA + LTE models in older generations that support only Verizon have a "VZ" tag instead of the "MM" tag.
Example: The 3rd generation iPad, the first iPad to come with support for both CDMA and GSM on a single device, has a CDMA/GSM/LTE Verizon model, but it does not support Sprint. So it's marked as "Cellular (VZ)" to signify that it's tied to Verizon (and cannot work with another CDMA carrier like Sprint).


In Apple's current terminology:
"Cellular" implies GSM+LTE
"Cellular (MM)" implies GSM+CDMA+LTE (across CDMA carriers)  

Note that there is no "CDMA only" version of the iPad after the iPad 2, unlike "GSM only" versions that have been and continue to be available.

You can come to similar conclusions by comparing the cellular support across iPad models/generations and how they relate to the chipsets used, the carriers supported, and their technology implementations.  

Here are some links related to the iPad specs across generations and multi-mode (as it applies to cellular technology) that would help understand my conclusion better.

Technical specifications for iPads of different generations (pay attention to the technologies/carriers supported):
1. iPad (4th Generation) - Technical Specifications
2. iPad mini - Technical Specifications
2. iPad (3rd Generation) - Technical Specifications
3. iPad 2 - Technical Specifications  

Links related to multi-mode (read them completely or search for "mode" to go to multimode related snippets on these pages):
1. Multi-band and multi-mode phones, Multi-mode and multi-band mobile phones
2. Cell phone bands and modes
3. Qualcomm Fifth Generation Gobi Platform
4. A rare look inside an LTE cell site
5. Sprint - multimodal hardware rollout
6. Intel previews multimode LTE chips