A shliach tzibur should be one who is familiar with the pesukim from Torah, Neviim, and Kesuvim that are so numerous in the tefilos, so as not make mistakes in pronunciation. There are poskim who say that even a temporary shliach tzibur should not be appointed to the position unless he is capable of articulating the words smoothly and correctly. Nevertheless, the custom is that for an occasional yahrtzeit or similar occasion we are lenient and allow people to go to the amud even if they are not very proficient in the pesukim. There are many reasons for this leniency. One is the difficulty of maintaining this standard, since many people have yahrtzeit throughout the year and it is very important for them to daven at the amud; we do not want to create distinctions among community members, causing unpleasant situations. Another reason is that nowadays the shliach tzibur is not actually exempting us from our personal obligation: each of us davens on our own, and consequently, there is some room for leniency. However, it is definitely preferable that the person who davens at the omud should say the words properly, without mistakes.