Is there an idiom or expression to say "when something is beyond your understanding, you think very little of it"?

B would like to tell A: Don't talk about things you don't know anything about. B may think that A is an ultracrepidarian, a person expressing opinions on matters outside the scope of one’s knowledge or expertise (used as a noun or adjective).

The site Wordhistories shows that:

This word was specifically invented to qualify the English poet and critic William Gifford (1756-1826). With reference to the fact that William Gifford had been a shoemaker’s apprentice, ultracrepidarian alludes to the remark “ne supra crepidam sutor iudicaret”, “the cobbler should not judge beyond his shoe”, attributed to the painter Apelles in response to criticism about the shape of shoes he painted from a shoemaker. This anecdote is the origin of the proverb Let the cobbler stick to his last.

This proverb is mainly American and it means:

people should only concern themselves with things they know something about. (Oxford Languages)

or

Do not advise about or interfere with matters of which you know little or nothing. (Free dictionary)

Here are some other funny options of what B may say to A, which may not cover precisely your meaning, but would work just fine in your context:

Better to remain silent and be thought a fool, than to open your mouth and remove all doubt.

A little knowledge is a dangerous thing.