Your question needs to be unasked a bit. Ain’t isn’t a mispronunciation of isn’t; it was originally a contraction of am not that was then generalised to all the negative forms of to be.
Prolly and its rarer cousin probby are derived from probably by phonological reduction, the process by which words lose phonetic complexity through omission, approximation, and transposition of sounds. By the same process, comfortable is usually pronounced comfterble, which can be abbreviated and diminuted into comfy.
Different kinds of reduction include cluster reduction (knife→nife) and vowel reduction (and→n).
This is one of the major reasons that the pronunciations of English words can stray so far from their written form—walked used to be pronounced as it is written, wall-ked, rather than the woct form we hear nowadays.
Unrelated to your question, the other major reason for the state of English spelling is a drastic change in English vowel pronunciation called the Great Vowel Shift that occurred between about 1350 and 1500. The pronunciation changed, but the spelling didn’t.