Yes, that would be a nice feature; the only problem with this is that servers provide you with a person's information, such as DNS (Ip address is gained through /dns <nick>) and most other info is in a /whois (such as idle time, away time, etc).
Unlike services, such as AIM where the information is readily available on Client-Side, mIRC must retrieve this information every, let's just say, millisecond in order to keep up with when you may actually right-click. Now, why can't i you might ask? Well, there are 2 explanations about this. One is that this can lag servers very bad. Imagine there are 1,000 users on every channel and 800 of them are using mIRC. Every users' mIRC updating every bit of info on every user every millisecond for every channel, nick and etc. You do the math, you can understand why this would cause floods. So say one user is on one channel, there are 40 people on that channel. This single users' mIRC is updating 40,000 times a second. That's bad :-)
The second thing is, you may be asking "How about just grabbing the info at 'real-time', or whenever a user points the mouse over and let's it hang, or, as you said, in a right-click. Simple: since the servers' information is only available at request, ie: you must ask the server for it before you can gain access to the information, you must do a few things: /dns <nick>, /whois <nick>, /ctcp <nick> TIME, and perhaps any other info you want to gain; next you would have to parse this information by waiting for the server to send it all back through a RAW response. After that, you'd have to have it gather this information through a filter, filtering out the unnecessary information, then displaying everything in a nice popup and or tooltip. Why is this a problem? Well, like I said, you have to request the data for it to be sent; so, of course, there is no way to gather the information at real-time, because the server has to send it when it can, not immediately. The information is just not readily available at all times.
This would be a great idea, but it's not at all practical at this point in time.