Mr. Pocket is a lecturer on “domestic economy.” Why is this ironic?From Chapter 33 of Great Expectations

That Mr. Matthew Pocket is paid for his advice on
"domestic economy" is humorously ironic because in his own household any suggestions
that he makes in this area are completely ignored.  In Chapter XXIII, for instance, Pip
is invited to dinner at the home of the Pockets and while there is amazed as the
child-rearing that is more of a "tumbling-up" than management. In this chapter, Pip
observes that Mr. Pocket can do nothing to control the chaos of his household as
children barely escape harm from falling and from nutcrackers.  But, while all this goes
on, Mrs. Pocket refuses any constructive suggestions from anyone.  Therefore, in his
frustration, Mr. Pocket puts his hands into "his disturbed hair" and appears to lift
himself up by it. Then, he quietly goes on with what he has been
doing.

In Chapter XXXIII, after Pip has visited Estella on
Richmond Street, he draws near to the home of the Mr. Pockets and points to the irony of
Mr. Pocket's being a domestic advisor,

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Mr. Pocket was out lecturing; for he was a most
delightful lecturer on domestic economy, and his treatises on the management of children
and servants were considered the very best text-books on those themes. But Mrs. Pocket
was at home, and was in a little difficulty, on account of the baby's having been
accommodated with a needle-case to keep him quiet during the unaccountable absence (with
a relative in the Foot Guards) of Millers. And more needles were missing than it could
be regarded as quite wholesome for a patient of such tender years either to apply
externally or to take as a
tonic.

Then, in the same
passage, Pip states,

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Pocket being justly celebrated for giving most
excellent practical advice, and for having a clear and sound perception of things and a
highly judicious mind, I had some notion in my heart-ache of begging him to accept my
confidence. But happening to look up at Mrs. Pocket as she sat reading her book of
dignities after prescribing Bed as a sovereign remedy for baby, I thought—Well—No, I
wouldn't.

The contrast
between the reality of the Pocket home and the content of the lectures that Mr. Pocket
so competently delivers is indeed sharp.  Hence, the irony
exists.