Both Aikido and Kuta are designed to deal with multiple attackers. For this reason if no other we definitely do not want to end up on the ground. Aikido has training from the Japanese sitting position which is on the knees. This is good because it is easier to get to this position from the ground to a standing position but it is not practicing what to do if you end up lying down on the mat. Jack has some ground fighting information in his Hikuta course that seems to rely heavily on Catch Wrestling. Knowing a few techniques to deal with a ground fight may be better than nothing but it will not carry one far in dealing with someone with just a few months of ground training. Some of my friends think that they will be just fine it they gouge eyes, tear lips and ears and strike genitals. This may be true but the ground fighter has probably given far move thought to these strategies than a stand up fighter. There is also a legal and moral issue involved in gouging out someone's eyes who is really just playing, holding you down. These dirty fighting techniques are good reasons for us not to take a fight to the ground but if one ends up on the ground having good sound movement could be the difference in life and death and the difference in harmless encounter and a long prison term for viciously handicapping someone. In addition to martial considerations knowing ground fighting, Katamewaza, is part of being a well rounded martial artist and it is too much fun to spurn.
Mat work is the same as standing. On the mat the object is to control balance. A pin is basically lying on top of your partner so that the partner cannot get up. The trick to getting up is to disturb your partners balance. Much of training should be learning these balance skills.
Mat work also involves joint locks and chokes for submissions. Beyond this one must learn strikes and kicks from the ground. Finally there are the dirty tricks. We want to always fake the dirty tricks first in practice so that we remember to think of them. Then we want to fake the strikes and kicks. In practice we do not want to use pain to set up our submissions or escape pins because the object of practice is to develop control of our balance and our partners balance. We also want to learn proper leverage to get the submissions. If we use pain in practice we might move our partner without technique.