So what if we have to mention someone in the text should we use Hashtag or directly tag him/her?

ASSALAMU'ALAIKUM,

Hi, It depends on what kind of a relationship you share with them and it also depends that what platform(social media) you are willing to use.

If you are good friends with them then using hashtag or directly tagging them won't really affect your relationship with them so there is no harm in doing so but if you are trying to maintain professional relation ship then it is good to tag them directly or leave a private message and in case you want to show off whatever you have done or you need some exposure then yes you can use #tags. Twitter is a medium where people use #tags for almost everything they want to describe so you can use  #tag on twitter irrespective of whom you are willing to tag. I hope this helps.