That is a common misconception. Many medieval artists were extremely talented, it is just that their art does not fit what modern people expect art to be. It is a bit like looking at Picasso's Les Demoiselles d'Avignon and asking if he was even capable of drawing a female figure or if he knew what it looked like. (He was and he did draw very realistic art in his younger days.) And what about Dali's The Persistence of Memory, why are the watches bent, and what is that weird creature in the middle, none of the painting looks like reality. Well, modern art is often not realistic, and that is on purpose. There is a lot of modern art that looks like a child could have made it, but that is not the point of it. It is about your perception of it, and perhaps about changing your perception of what is art and what gets represented. So it is with medieval art.

Medieval art, especially art from the early medieval period, was not at all about representing reality in a realistic way. It was about symbolism and representing abstract concepts. Take for example the sculpture of Portrait of the Four Tetrarchs, which is a sculpture from the late Roman Empire but that is an excellent example of the trends of the medieval art. It is carved out of porphyry, a very hard material to work with, which was associated in antiquity with royal power. The four figures look nothing like the actual emperors, but they are symbolic of the power of the Roman Empire and of each of the emperors. The cloak, the crown, the sword with an eagle head for a pommel, all are symbols of the power of the office of the emperor. The beard on two of the figures represents the seniority of the two senior emperors, regardless of the actual facial hair of the real senior and junior emperors. The sculpture was made by a skilled artist for public display in an important building in Constantinople. It is not realistic, and it is not realistic on purpose.

Religious art of the middle ages (and nearly all of it was religious) was trying to represent an abstract view of the kingdom of heaven, of the power of God and Christ, and of the power and meaning of the divine. It was about the reaction of the viewer, usually a religious person versed in the meaning of the symbols used in art. It was a bit like the Tamarian language in the Darmok episode of Star Trek: The Next Generation, where you could understand the meaning of what was said in Tamarian only if you had a previous knowledge of the allegories used. Medieval art users would know the iconography that assigned (as an example) animals as symbols of the four evangelists, Mark as a winged lion, Luke as a winged ox. They would be familiar with Revelation and Exekiel where those symbols came from, and would understand the meaning of the symbols without needing to see a realistic depiction of the evangelists. A fish or an anchor would be symbols representing Christian followers in early christian art. Depictions of heaven would have a flat gold background, the blessed would be surrounded by an oval mandorla (like in this Christ in Majesty that shows Christ surrounded by the symbols of the four evangelists), and the size of a figure would represent its relative importance. All those were "Tamarian language" allegories that a medieval viewer would have recognized immediately.

The level of complexity in some of the medieval art is amazing. Take for example the Book of Kells, which is a masterpiece of illumination. The more lavishly illuminated pages like the incipit of the book of John showing the evangelist, once again with the Eagle as his symbol) are essentially illegible to an untrained eye, the evangelists are represented by their symbols, which are even harder to recognize than in other medieval art, and Christ and other human-like figure look weird, isolated and oddly proportioned. But if you were able to zoom in to any of the abstract details in the illumination you will find incredibly minute geometric patterns, as well as snakes, dragons, horses and other weird creatures, all abstract, and in such a fine detail that it is hard to comprehend how they were possibly painted with the tools and materials of the era, and without magnifying glasses, under the natural light of cloudy and gloomy Scotland.

So if you see medieval art that does not look like what you would expect, that is different from the art that preceded it and that that came after it, admire it for what it was meant to be: a more or less abstract depiction of abstract religious concepts, often in a simple looking but often with extremely complex detail.

In addition to the primary sources that I linked to above (and the actual paintings and sculptures are always excellent for appreciating and understanding art), Lawrence Mees, Early Medieval Art in the Oxford History of Art series is an excellent introduction to medieval art and its meaning, a book that explains a lot with simple language (and lots of great images) that don't require the reader to have previous deep knowledge of art history.