Mueller is:
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Not pissed at Barr's conclusion, but stated that it doesn't totally encapsulate the efforts of the SC investigation/report. This is a true statement as the summary only addresses collusion/obstruction questions.
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Is pissed at Pelosi's assertion that Barr is lying/covering for the President. This implies that Mueller knows that Pelosi is making false statements based on what he'd assume she should know (at least by now).
I'm going to go out on a limb here and say that Mueller's response to Barr's assessment in #1 is directly related to the 6% redactions because Barr's summary doesn't mention anything about those redactions outside of "They are redacted due to ongoing investigations/grand jury proceedings". That covers that. The reason I say that is because Barr didn't fail in his summary to state the conclusions in the report, that, although there were "things and stuff going on with Russians", none of it was obstructive.
Another theory is that Mueller's response to Pelosi in regards to #2 is because Pelosi and the Dems are running around with their hair on fire, and making erroneous allegations towards Barr (kabuki) because they are deathly afraid of what is actually in the 6% redactions, and refuse to address them in the context of the ongoing public conversation, as it relates to the report's conclusions.