Learn English – n idiom for “making a puerile excuse”

In English, we have the infamous know-it-all: one who knows everything; hence, a person who makes pretension to great knowledge, especially one whose didactic conversational habit conspicuously reveals his belief that he has superior knowledge on many subjects; a wiseacre; a know-all; -- usually ironical. [Colloq. & pejorative] Note: the use of this term implies that the speaker disapproves of this behavior, and may think that it is unjustified.

**know all: someone who seems to know everything and annoys other people by showing how clever they are.

No one likes him because he's such a know-all.

smarty-pants is an older idiom, for one who is obnoxiously self-assertive and arrogant, as is weisenheimer.

There is the Jerkass, who might say something like, "Sometimes I park in handicapped spaces while handicapped people make handicapped faces. I'm an asshole!" (— Denis Leary, Asshole) but this is more trope or jargon.

And, as StoneyB has kindly reminded me, there is the loudmouth:

Be loquacious, often noisily or boastfully; someone who talks too much or too loudly, esp. in an offensive or stupid way

*Harvard Square: Know-it-all capital of the universe. * - Universal Hub.

If you mock someone’s misery, you rub salt into their wounds, described in the Oxford Learner's Dicionary as:

to make a difficult experience even more difficult for somebody

This Dictionary of Idioms gives a fuller definition and example:

Rub salt into the wound/(someone’s) wounds to make (someone’s) sorrow, shame, regret etc worse, often deliberately: I was very disappointed at having to miss the concert, and my friends kept rubbing salt into the wound by telling me how good it was.

And two examples from the media, where rubbing salt into the wound specifically means mocking someone’s misfortune:

City fans mock Manchester rivals: [Manchester] United let a 3-1 lead slip to lose 5-3 at Leicester yesterday, before City took on Chelsea at the Etihad [stadium]. And cheeky Citizens fans took the opportunity to rub salt into their rivals' wounds with an aerial banner. “United we fall ― in cinemas soon," read the message flown over the Eithad.

Hilarious Comeback Joke Takes Off On Reddit After English Mockery: Last week, rugby fans around the world cruelly wallowed in England’s early World Cup exit. Several jokes were posted online and it seemed as if England’s closest neighbours were most keen to rub salt in English wounds.