does any one have idea, why temperature is increasing to that extend?
Either something else in the environment is heating up very slowly this way or something is wrong; the third load average average number (presuming
htop doesn't do something bizarre, which I don't think it does) is over 15 minutes, and it is higher than the first one, which is the past minute, meaning this pi has not been very busy for that period1 -- which should be plenty of time for heat to dissipate from the SoC unless, e.g., it is encased it with some kind of spray on insulation. In fact, I am sure I have read about people sealing the whole board up with thick plastic film for use underwater and not report overheating problems (the water may help with that, but I really think the manufacturer is telling the plain truth when they say it does not require any cooling or ventilation).
Even if it were working 100%, I think a Pi 2 can sustain that indefinitely (presuming again it is not in a very constrained, airtight enclosure), without heatsinks or much ventilation, in 20-25 C air without ever going over 60 C; when I've run one that way it plateaus in the low - mid fifties after a few minutes then stays there. Of course, if you overclock it sufficiently, at some point that behaviour will start to change.
This doesn't seem necessary just for a wifi dongle, so you could try setting that back to 0. I do have an TP-Link RTL8188CUS based adapter which I used to use with pis but stopped because it had some unusual disconnection glitches and also got ridiculously hot (although this did not affect the SoC). So if you can physically inspect the pi, you might check that, since it is probably indicative of an excessive amount of current flowing. Other adapters I've used did not have this problem.
For comparision, the B+ I run 24-7 in a very poorly ventilated (plastic case inside a wooden box) 20-25 C environment currently has a load average of
0.0 0.1 0.3, an uptime of ~16 days, and a core temp of 42.7 C. It also has
max_usb_current set in order to power a USB drive, with a total 2.1 A supply. Sometimes I've used a 2.5 A supply, I've been doing this for years, and I occasionally have uptimes of 5-6 weeks without noticing anything like this (I do like to prod and check things). I've done the exact same thing with a B, although I haven't used a 2 in that role.
In short, this is not what should happen.
1. You also have a very small cache (the red bars at the end of the memory line), another clue probably nothing serious has happened for a long time. By comparison the B+ in the last paragraph is currently using less RAM (73 MB) but the rest of the 1/2 GB is all red bars (i.e., occupied by cached data) which is what usually happens with long running systems unless they are dedicated to a very small number of specific, light duty tasks. Not an absolute, but a significant sign.