These two possibilities are not mutually exclusive.
While your guess about the thought processes of the originator may well be correct, it is still the case that the result, "Mongita", is ① unambiguously Spanish and ② unambiguously pronounced in Spanish as [monxita], which is a real Spanish word, the diminutive of the common word monja, meaning "nun". But [monxita] is spelled "monjita".
The result is that what may well have been an incorrect application of the diminutive suffix (the correct result would be "Monguito") produced a misspelling of "monjita". It's just as clearly misspelled Spanish as "Ke keres aser?" or "yerba maté", if not more so. So you can expect most Spanish speakers to read it as ridiculing the literacy of an unspecified person—more so if they also know English, given that "mongo" has been an English word used for ridiculing someone's intelligence for many generations.
While your guess about the thought processes of the originator may well be correct, it is still the case that the result, "Mongita", is ① unambiguously Spanish and ② unambiguously pronounced in Spanish as [monxita], which is a real Spanish word, the diminutive of the common word monja, meaning "nun". But [monxita] is spelled "monjita".
The result is that what may well have been an incorrect application of the diminutive suffix (the correct result would be "Monguito") produced a misspelling of "monjita". It's just as clearly misspelled Spanish as "Ke keres aser?" or "yerba maté", if not more so. So you can expect most Spanish speakers to read it as ridiculing the literacy of an unspecified person—more so if they also know English, given that "mongo" has been an English word used for ridiculing someone's intelligence for many generations.