MacBook – “QuickLookUIService” Using Significant Energy

I am using a 13-inch mid 2014 MacBook Pro with Retina Display. It is about 3 weeks old and already has clocked 13 charge cycles. Is this too high?

A charge cycle is just the process of using and then recharging the full capacity of the battery one time. From Apple:

A charge cycle means using all of the battery’s power, but that doesn’t necessarily mean a single charge. For instance, you could use your notebook for an hour or more one day, using half its charge, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two, so it may take several days to complete a cycle.

You can't really have "too many" charge cycles unless there's something wrong with the battery and it's unable to hold a charge. That seems unlikely since your computer is brand new (and OS X will notify you if your battery condition is abnormal).

Furthermore, your particular model is rated for 1,000 charge cycles before it's considered "consumed" (meaning it'll no longer hold a reasonable charge and will likely need to be replaced). If your current rate of 13 charge cycles per three weeks holds, that means you're good for 4.5 years – in other words, nothing to worry about!

Also, I follow the practice of fully charging it and then using till about 40% before recharging without ever leaving it plugged in. Is this the best practice to follow or should I leave it plugged in while charging ?

Even if you leave your laptop plugged in at 100%, the system will actually silently discharge the battery to ~95% and back up to keep things moving. Apple used to recommend unplugging your laptop at least once a month, but their current guidelines make no mention of that, so I wouldn't worry about it.

I'd simply ignore it. Energy Impact is just an arbitrary number showing how much energy an app is consuming (each instant and on average).

Activity Monitor is simply telling you that the app which uses the most energy is Safari.

To make sure that Safari is not eating up on CPU, you can check the CPU usage over time, for example using a tool like 'uptime' from the command line (simply launch the Terminal an type uptime), this will tell you the uptime of the system and the load average over the past 1, 5 and 15 minutes, this number should be close to (or less than) 1 on Yosemite. A load average constantly above 2 (or more) could indicate some process sucking up on CPU, which can found using Activity Monitor.