Is good good

Origin of the idiom “The opposite of 'good' is 'well meant'.”

As @splattne wrote correctly, the quote is complete

The opposite of Well is not angry, rather well-meaning.

In addition to Brecht and Tucholsky, Erich Kästner (1899-1974) is sometimes named as the author and more often Karl Kraus (1874-1936). But nowhere did I find an exact reference to the source.

In the case of Kästner, I would rather suspect a mix-up with his saying

There is nothing good, unless you do it. Source: moral in: Doctor Erich Kästner's lyric medicine cabinet

The saying comes from Karl Kraus

Evil never thrives better than when preceded by an ideal. from: aphorisms

The idea is pretty close. He would therefore certainly be plausible as the originator of the quotation sought, but this could also be the cause of a wrong attribution.

Kraus, too, only modifies an older idea (here first introduced by @ marton78):

The road to hell is paved with good intentions.

Wikipedia names several possible authors and leads the line even further back into the 11th century to Bernhard Clairvaux ("L'enfer est plein de bonnes volontés et désirs" - "Hell is full of good intentions and wishes"), to Virgil and into the New Testament.

@splattne pointed out Gottfried Benn. According to this blog post, its version is

Wherever there is thought, Germanness already feels betrayed; but wherever art appears completely endemic, it sends the pharmacists to the front. A fine instinct, by the way! The word got around that the antithesis of art is not nature but is well meant; Style is a malignant neoplasm, a lethal one.

The source is named there: "Gottfried Benn: Roman des Phenotyp (1944); towards the end of the section“ Static Metaphysics ”; in: Gesammelte Werke, edited by Dieter Wellershof, Frankfurt aM: Zweiausendeins 2003, Volume 2, p. 1333 f. ", so this quote comes from a time when Kraus and Tucholsky were already dead.

As we have seen, the basic idea that good intentions can have bad consequences is much older. Benn expresses himself in a similar way specifically on the relationship between art and nature. His quote is the only documented source that uses the phrase "well-intentioned" and emphasizes an alleged contradiction, but it is clearly not the phrase sought. It is theoretically conceivable that Brecht or a stranger took up this saying and generalized it to the quotation sought. But it seems more probable to me that the sentence I was looking for was coined in the 20s / 30s of the 20th century by Brecht, Kraus, Tucholsky or an unknown who faded behind these names and was modified by Benn for his book.