Is this frequency showing correct on crt?
I play my mister on my 21'' trinitron via component and just realized on this screen that it shows frequency at 59.8Hz.
Is this correct or somehow any mister settings was changed to display it like this?
That's correct. Totally normal. In this menu the first is the native resolution and the second is the scaled resolution. The Menu core (what you see upon boot) is a bit special and different, but 59.8hz is normal for analog video. Cores of different systems will have different native refresh rates that vary widely so you will see this number change around a bit. 
Also, where do I get a 21 foot Trinitron? Sign me up! 
The exact "standard" frequency of NTSC is 59.94Hz, but virtually any device (and in the case of MiSTer, any core) will deviate slightly from that spec depending on their output resolution and other timings. Which is fine and intended, because CRTs are designed to have a certain amount of leeway.
It is one of the major advantages of MiSTer is that it can exactly reproduce the output frequency of any platform without any fuss.