Grrr.
No it isn't.
Yeah well, you can guess there was an earlier write-up here once...
In Latin, ignoramus doesn't have a plural, because it's a verb, first person plural, present tense, and shoving the grammar aside it means precisely we don't know. You can't have a multitude of we don't know. It's a complete sentence, used in old law: see Webster 1913 on ignoramus, first entry.
In English, ignoramus has been converted into a noun, 'ignorant person', so it can take a plural. But it has to take the English plural -- ignoramuses -- since there is no such thing as a Latin plural of it.
It is worth emphasizing that there is no rule in Latin grammar that says change -us to -i to make a plural. That's not how Latin grammar works. It all depends on part of speech and word class (declension) within part of speech. The -us -> -i subclass is quite numerous, that's all. But you can't assume that if a word ends in -us it belongs to ths subclass. (Others: corpus becomes corpora, opus becomes opera, apparatus just lengthens the vowel to apparatûs.)
The Latin verb stem is ignor-, from which of course our word 'ignore' comes, but the Latin is a single word meaning 'not know': ignoro 'I don't know'. It comes from the negative prefix in- 'un-' plus the root gnor- (older gnos-), related to English know and to the Greek stem in gnostic, gnosis, gnome and even to the Sanskrit jñana, a kind of yoga.
(Of course you could also combine separate elements for 'not' and 'know' (scio), and this gives our word nescient 'ignorant', which gave rise to 'nice', which originally meant the same thing, viz 'ignorant'.) I've finished now.
Harumph.