That seems pretty hot. My Pi 3 with everything overclocked and no heat sinks runs under 60C. Do you see warnings in the upper right corner of the screen for temp?
A few things:
1. most heats sinks for the Pi are a joke and don't use thermal paste, they just use stickers. In this situation you would be better off without them.
2. does your case have any vent holes?
3. what are you using for your OS base: raspbain, retropie, diet-pi? Maybe you have more running in the background then you need.
4. what layout/theme are you using? Some might be more intensive then others.
5. are you using hardware video decoding? If not, the CPU will be over worked.