Well 880M runs hot for 2 reasons:
1) It's a Kepler architecture
2) It's an overclocked 780M
That being out of the way the temperatures you are getting are considered normal. People achieve (I can vouch for me and others that I've read of on this forum and others) 90°C+ quite easily as of the above stated reasons. The card won't go above 93°C as that's its absolute maximum it can go and beyond that it starts to lower the clocks in order to achieve lower temperatures.
People getting 70°C on overclocked cards get that because of reasons such as:
1) They're lying
2) They're using newer (Maxwell) or older GPU's that have lower clocks to start with and produce a lot less heat.
3) They are lucky. The silicon on their GPU is really special and gives them extra ability to OC it.
4) They have repasted the GPU which gives them lower temperatures but only for some time as the difference in a big run isn't that big (5-8°C)
Now some temperatures I've achieved on stock clocks on my 880M:
Assassin's Creed: Unity - In menu it reaches 93°C in few seconds for some reason, otherwise it howers around 85-90°C
Counter Strike: Global Offensive: Exactly 69°C
Crysis 2: Around 80-85°C
Far Cry 4: Howering around 90°C
GTA V: It can get as high as 93°C in intense areas after long play sessions and as low as 85°C
DayZ: Around 85°C
As games are progressing in graphics you'll be reaching 90°C more and more and that's nothing to be afraid of. The card is OC by default and using Kepler architecture which is already running hot by itself. With that being said, there's nothing more to say from my side for now.
Cheers
Asus Rog G750JZ
Screen: 1920x1080 17.3'
CPU: Intel® Core™ i7-4700HQ (3.4Ghz @ -70mV)
GPU: GeForce GTX 880M 4GB (OC +125Mhz Core, +600Mhz Memory @ 1.062V)
RAM: Samsung 32GB 1600MHz
Storage #1: Samsung Evo 840 250GB
Storage #2: Samsung Evo 850 500GB
Storage #3: Seagate Barracuda 2TB
Storage #4: Samsung Evo 128GB MicroSD