what does A64 with DDR3 bus mismatching cause?
Based on my own experience designing and debugging issues with DDRs and processors, these are very nontrivial circuit designs that you should probably stick to a reference design. Usually the chip manufacturers have an application note with some guidelines you need to follow in order to get a working design.
Now here are a few things you need to keep in mind:

I have already read a lot about mismatching problem, and I need to setup the layer stackup to get the required matching impedance

For the clocks, which are differential lines (CLK_P &amp; CLK_N), and really all of the data lines, not only do you need to worry about impedance but also length matching. That is, the actual physical lengths of the traces from the CPU to the DDR need to be the same length to avoid problems with signal delays, etc. At the speeds these DDRs work, this is an important thing to keep in mind.
The clocks are usually specified to some impedance (e.g 100 Ohms differential) but make sure you add series resistors in each of the clock paths, for damping purposes (could have issues with regulatory certifications). In a recent board I was debugging, I found that I could fix the boards that weren't stable over temperature, by adjusting the value of those resistors. In a nutshell, some chips are kind of marginal when you produce thousands of them.
(we build about 20k boards with DDR parts per month and about 10 or so boards showed this one issue. Not a big deal now because the percentage is rather low, but something we had look into to avoid having this turning into a bigger problem).
Here is what length matching looks like:

DDR Routing
Another thing to keep in mind (as important as anything) is all the bypass capacitors around the DDR. These are critical, you have to make sure you have enough of them and place them accordingly to the design recommendations.

does mismatching cause this temperature rise?

I doubt mismatching is going to have a significant effect on the temperature rise. What I feel makes a considerable impact, is how fast (e.g data rate, clock speeds) you're running the DDR. In the datasheet, you should be able to find the temperature characteristics of the part you're using. Like I mentioned before, DDRs are really finicky and very sensitive to temperature, that's why you should probably stick to a reference design or follow the recommended guidelines. It can take more than one board spin to get a design that works well over temperature and remains stable.
Most DDRs I've worked with seem to perform better at higher temperatures (within the specs) and start to act up at low temperatures (you need some type memory test, eg. 'memtest' to stress the DDR). This is something you need to qualify in the lab, in a temperature chamber, and should give you more insight.