Are DNA based vaccines , or RNA based vaccines ,"better" , for developing vaccines ,able to fight a mutating virus like Covid ?

Neither technique is superior for addressing the issue of mutation.

Well, first of all, the virus causing COVID is an RNA virus, not a DNA virus, so a DNA-based vaccine wouldn't make any sense in this case. 

But in principle, even with a DNA virus, it'd make more sense to use an RNA for the vaccine, since by bypassing the time-consuming processes of transcription, RNA processing and transfer out of the nucleus, it'd accelerate production of the antigen you want to make antibodies against.  Also, with an RNA reagent, there's little chance for affecting the cell's nuclear genome in some unpredictable way.  RNA is more targeted and temporary--once it's done its job it's rapidly degraded.

With respect to he issue of mutation--that is always a potential issue.  However, a big part of vaccine development is to try to use antigens that are not known to undergo significant mutation, e.g. use an antigen that is so essential for viral function that it is generally not "free" to mutate as readily as some other one that is under less selective pressure.  That's easier for some viruses, harder for others.  I don't know yet what the tendency is for spike protein mutation.