Yes, you can still do magick while Mercury is Retrograde. 5 of the 7 traditional planets go retrograde, and each of the planets govern particular areas of life. Now, retrograde is traditionally a minor afflicted, it is still an affliction and not ideal, but it does not compare to being afflicted by Fall or by Detriment, yet new agers don’t “feel” when Mercury is in his Detriment or Fall, now do they?
If you perform magick for business, or communication, then doing this while Mercury is retrograde will weaken the magick and delay the outcome. But, Mercury is not the only planet that is to be considered, there are many other factors present at any given time, such as the planetary day and hour, the phase of the moon and the sign she is and the aspects she makes and the house she is in, and then the lords of the appropriate houses, and so on.
With that said, timing magick by astology can be beneficial, as performing magick when everything is extremely ill for the magick performed, then the outcome will only arrive after great pains and suffering, and great delays, particularly if the spirit called on is of an especially violent nature. But, it is rare that magick is performed under such inauspicious times without specifically choosing them. So, I don’t generally go too complex for timing my magickal rituals or prayers, except for in the creation of astrological talismans, in which case it is necessary.
If a case is urgent, I simply time by an appropriate planetary hour, and repeat the ritual to strengthen it, and this can work wonderfully by itself.
But, it can be looked like this: When you plant a seed (and I am speaking literally here) under a very Astrologically auspicious time, the seed will germinate if it is possible to germinate, and the planet will grow to be wonderful, healthful, growing quickly, without trouble, and producing a wonderful harvest. But, when it is planted while under a very inauspicious time, it will struggle, face many challenges and struggle much, quickly decay, and die very quickly, if the seed even germinates at all. And this I have seen plenty of times.
It is the same for any action that can possibly be done, and magick is no exception.