"No man is an island, entire of itself; every man is a piece of the continent, a part of the main.
If a clod be washed away by the sea, Europe is the less, as well as if a promontory were, as well as if a manor of thy friend's or of thine own were: any man's death diminishes me, because I am involved in mankind, and therefore never send to know for whom the bells tolls; it tolls for thee."
My above quote referencing the use of diminish is from "For whom the bell tolls" written by John Donne, who lived from 1572 to 1631.
"Diminish" is rarely used in this context today, although it is valid.
I would write your second sentence as
I would never mock you or
put you down .
Or even I would never mock you or
disrespect you .
ell.stackexchange.com/users/56820/Weather Vane
The most common way of phrasing this would be to include the verb "try":
I would never mock
you or try to diminish you.
This is a fairly common usage, which sounds like what you want to say.
Of course you can say it without the "try" but that implies that you actually have the ability to make the other person "less" simply by making fun of them, so it would be a rude/insensitive choice when you want to be nice.
Some similar examples from around the web:
“They tried to diminish me and Joe , and the whole point of their character assassination was to divert attention from the real issues.”
—
Valerie Plame, quoted in "Valerie Plame Wilson: the housewife CIA spy who was 'fair game' for Bush",
The Telegraph , 15 Feb 2011 When someone curses you out or insults
you profane language, he is trying to diminish you .
—
Stuart Schneiderman, "The Civil and the Profane" blog entry, December 19, 2013
You’ve had more than enough of those who tried to diminish you .
These guys always tried to be above you.
They insulted you, they patronized you and diminished you in all possible ways because that was good for their egos.
—
Selma June, "A Man Who Thinks You Are Worthy Will Come Along", Her Way , March 09, 2018
ell.stackexchange.com/users/37569/1006a
I think there is a difference.
Mock, belittle, disparage, disrespect, put you down, diss:
these are all things I can do to make you seem less worthy than you are.
Diminish, in the sense Donne uses it, means you actually become less worthy.
"
For all his greatness however, he was diminished by his last years and by Churchill's disastrous decision to recall him to the Admiralty soon after the outbreak of the First World War. "
- book review on Amazon
"
He was able to handle Martinez with ease — yes, he was diminished, but no, that didn't stop..."
- article discussing whether a boxer is likely to win his next contest.
These quotes use "diminish" to mean "become less".
It is harder to find uses where it means "to make less", and where the subject and object are both persons ("I would never diminish you").
In none of these examples is someone diminished merely because of what people say about them.
ell.stackexchange.com/users/9574/Michael Kay
Hmmm ...
By a dictionary definition it sounds valid, but I've never heard a native speaker say that.
People will say "diminish your reputation" or "diminish your status", but not simply "diminish you".
I wouldn't say it's wrong, and people would understand what you meant, but it's just not how people commonly use the word.
ell.stackexchange.com/users/803/Jay
In terms of grammar your sentence makes perfect sense.
It is possible to diminish a person.
Generally however, an english speaking person wouldn’t use the word diminish and would use something else.
Maybe replace it with something more common like “shame” or “tease”.
If you want the meaning to stay the same but sound better try “belittle”.
ell.stackexchange.com/users/76669/Kuzuno
"I will diminish and go to the west" --Lady Galadriel, The Lord of the Rings , J.R.R Tolkein
Using this as an example, I'd say it's acceptable if you're a writer to use the word this way, if what you intend is to create a very formal (archaic) character.
That said, there is some cross-over between the ideas of archaic English, and more "high" English among intellectuals who aren't exactly concerned with what is used in laymen circles.
ell.stackexchange.com/users/56707/Adam R. Turner
diminish might be used in that manner in a book about interpersonal relationships intended for lay audiences ("popular psychology") or in a psychology text.
ell.stackexchange.com/users/10264/Tᴚoɯɐuo
I would never mock or belittle you.
I would never mock or disparage you.
belittle
ell.stackexchange.com/users/33113/Lambie
In the following dialogue, is the second sentence correct?   "Thank you for not making fun of me."   "I would never mock you or diminish you."   I checked the Oxford dictionary and it defines  diminish  as  Make or become less .  Can I use the word in reference to a person in this context?
Is it okay to use the word "diminish" for a person?
ell.stackexchange.com
<word-usage>
ell.stackexchange.com/questions/169185
ell.stackexchange.com/users/76667/Clarence K.
It is acceptable to use the word "diminish" for a person.  "Diminish your reputation" or "diminish your status" are also used. This is a valid term meaning to put someone down, although it is not very commonly used by native speakers. Someone could accuse another person of trying to deminish them, but "diminish their reputation" could be more common.
This is a valid term meaning to put someone down, although it is not very commonly used by native speakers. Someone could accuse another person of trying to deminish them, but "diminish their reputation" could be more common.