Every visit to the cinema, despite the utmost watchfulness, leaves me dumber and worse than before.
Based on context, it seems that Adorno felt that "visits to the cinema" where a species of "idle chatter" (to use a Hedeiggerian expression). And thus he felt that they dumbed people down because they involved the use of language in a way that did not accomplish any sort of meaning discourse.
From the same Minima, he includes several other examples:
The overarching theme is the loss of horror, i.e., criticality towards the world. The cinema is dumbing experience for him, because even if the dialogue were appropriate given actual critical agents having a conversation, its display is no longer critical but rather encourages a sort of bourgeois complacency.
In The Culture Industry: Enlightenment as Mass Deception
They [sound films] are so designed that quickness, powers of observation, and experience are undeniably needed to apprehend them at all; yet sustained thought is out of the question if the spectator is not to miss the relentless rush of facts.
Even though the effort required for his response is semi-automatic, no scope is left for the imagination. Those who are so absorbed by the world of the movie – by its images, gestures, and words – that they are unable to supply what really makes it a world, do not have to dwell on particular points of its mechanics during a screening.
It seems he's saying that a movie goer understands a movie if they apprehend all the details and facts that make it up: and that there is no nuance to its structure:
no room for imagination or reflection on the part of the audience, who is unable to respond within the structure of the film, yet deviate from its precise detail without losing the thread of the story; hence the film forces its victims to equate it directly with reality.
Which is because a film
is intent upon reproducing the world of everyday perceptions
But why think that its supposed ability to reproduce life, defines all films? I suppose that's because of the nature of autonomy in capitalism - that it is a historical quality of art.
i.e. how can cinema be art if it was not already a potential in it.
The whole passage appears to be a meditation on certain aspects of 'sociability'; one might say the banality of sociability to paraphrase Arendts famous usage of the term; and given Adornos take on it - the 'evil principle' that had always 'lurked on affability' develops.
The passage begins:
Doctor, that is kind of you; nothing is harmless anymore
Simple civilities, which express an attitude of courtesy, and are 'harmless'; are in fact more, 'are the small joys, the expressions of life' and are thus 'exempt from the responsibility of thought'
Even the tree which blooms, lies
This image seems startingly similar to that which opens TS Eliot's Wasteland:
April is the cruellest of months
Adorno goes on to say:
There is no longer any beauty or consolation, except in the gaze that goes straight to the horror, withstands it, and in the undiminished consciousness of negativity, holds fast to the possibility of that which is better.
And malignity which was once
limited to toasts of cozy sociability ... has long since spread to friendlier impulses.
To which he gives two examples:
A chance conversation with a man on a train, one acquiesces to avoid a quarrel
Every visit to the cinema ... Leaves me dumber than before
Presumably one goes to the cinema in company; and afterwards one might discuss it.
All this he ties to a certain notion of sociability, the 'chatter' of @Virmaiors answer; for he adds:
Sociability itself is a participant in injustice, insofar it pretends we can still speak to each other in a frozen world.
ie nothing is really said, and thus the world is 'frozen'; and the 'flippant, chummy word contributes to the perpetuation of silence' in this image of affable sociability.
As the quote from the post shows, Adorno does not say that he hates the cinema, but he criticizes it. Just before the quoted passage he says
no thought is immune against its communication, and uttering it at the wrong place and in the context of a false agreement is enough to undercut its truth.
For me the whole passage from Minima Moralia sounds a bit complaining. As if Adorno were not able to separate a thought from the context in which it is uttered. Being overwhelmed by the context makes it impossible for him to receive the pristine thought.