Nobody has a right to any ministry at Mass save for the most important one which is to pray the Mass and offer up your own sacrifice. Nobody has a right to be an altar server, EMHC, or even reader. It is never recommended that someone be given a ministry to perform who can’t quite do it in order to be “fair.” When the ministry becomes about a certain person, it is their mishaps or inadequacies that becomes the focus and the prayer of the congregation suffers.
So, for example, in the absence of an installed lector, a reader may do the readings. But, according to paragraph 101 in the GIRM, this person should be, “truly suited to carrying out this function and carefully prepared.” This is so that when a reader reads, he or she does not become the focus of attention but that they become a finger pointing toward God, or, as the GIRM itself says, this is so that the congregation, “by their hearing the readings from the sacred texts . . . may conceive in their hearts a sweet and living affection for Sacred Scripture.”
Of course, there is a certain amount of leeway here. There is no universal test to be a lector. Everybody has an opinion. Readings should be more/less dramatically read. They should be read more quickly/slowly. There are considerations to be weighed such as how adequate of a sound system, if any, is used, how echoey is the space, to what group of people is the reading being proclaimed. Absolute rules are difficult to put down for universal observance other than, “suited” and “prepared.”