By using the absolute degree venustī, doctī to describe a group of people, we say that they're more beautiful or learnèd than most others. The others may be beautiful or learnèd in their own right, but the beauty or learning of this group of people just stands out to us as setting them apart from everyone else, and this prompts the epithet.
By using the comparative degree venustiōrēs, doctiōrēs we say that they're relatively more beautiful or learnèd than others, but not necessarily deserving to be singled out as such among everyone else in consideration. We simply regard them as "more or less beautiful or learnèd, rather beautiful or learnèd than not". At least that's the interpretation in this context - it will mean "the more beautiful or learnèd" if the context is picking and choosing people who exhibit relatively more of a particular quality, i.e. the reference is definite.
By using the superlative degree venustissimī, doctissimī we say that even in a group of people ordinarily regarded as beatuiful or learnèd, these particular individuals stand head and shoulders above the rest - they're the elite.
Catullus is saying that everyone who would like to think of themselves as even remotely "beautiful people" - a reference to his own in-group used in the sense of primarily spiritual beauty that's seen as holistic, a usage that also exists in English - should weep at hearing the terrible news, and preferrably be moved to memorise and recite the poem to as many other beautiful people as they can find.
While the comparative is more or less forced by the meter, using the absolute degree wouldn't work with quantum because it would imply a defined and extant group of people as opposed to meaning "anyone whatsoever who fits the description".