Ellicott:
A mixed multitude went up also with them.—Nothing is told us of the component elements of this “mixed multitude.” We hear of them as “murmuring” in Numbers 11:4, so that they seem to have remained with Israel. Some may have been Egyptians, impressed by the recent miracles; some foreigners held to servitude, like the Israelites, and glad to escape from their masters. It is noticeable that the Egyptian writers, in their perverted accounts of the Exodus, made a multitude of foreigners (Hyksôs) take part with the Hebrews.
Exodus 12:38, did the non-Israelite multitude (עֵ֥רֶב) cross the sea without the blood applied?
Some probably did.
Did this rabble (אֲסְפְּסֻף) in Numbers 11:14 instigate the unbelief in God's provision?
Not necessarily.
These are two different Hebrew words,
Cambridge:
a great mixed multitude] cf. Numbers 11:4 (the Heb. word different). Non-Israelites (cf. the same word in Nehemiah 13:3) of various kinds are meant: e.g. Egyptians who had intermarried with Israelites (cf. Leviticus 24:10), other Semites who had found their way into Egypt, and prisoners taken in war who had been employed in the corvée (Exodus 1:9).