What was the Soviet reaction to the revelations about secret US programs like Projects ARTICHOKE, MKULTRA, MOCKINGBIRD, etc., revealed in the 1970s?

Soviet modus operandi

First thing to consider is did the Soviets know about these experiments before things went public. Soviet intelligence agencies like KGB or GRU could have known something due to their wide agent network. But if they did, they may not have gone public with that knowledge, most likely to protect their agents who may have been still alive and living in the US, or have close relatives there.

Second thing to consider is what would they have done with such knowledge? The Soviets could have been interested in the results of these experiments in order to know how to counter possible effects or even employ them themselves. Of course, there was always the possibility to embarrass opponents and show to the world how evil the US and its allies truly were. In that case, the Soviets would not directly present evidence (let's say by spokesman of Foreign Ministry). Instead, again to protect their agents and to appear shocked as everyone else, they would leak incriminating data to the Western press. The Soviets did have informal contacts with several journalists ready to use anonymous sources, for example it was long suspected that Seymour Hersh actually got in contact with Soviet informants when he blew a lid on CIA Family Jewels.

The third thing to consider is what would the Soviets have done when the information became public. The Soviet government wanted to preserve an aura of reliability, especially when addressing a foreign audience. When addressing the Western public, they especially would use something already known to them or easily verifiable. But even in this case they would have a two-pronged propaganda strategy. If they wanted to point out something that the USSR was not doing, they would simply go ahead and tell that straight away. For example, they accused US many times of bombing civilians in Vietnam, including using inhumane ammunition like napalm against children. At that point of time the USSR was not engaged in open wars, so they used opportunities to portray Americans as murderous barbarians - and it was pretty successful.

However, if they had information about some nefarious acts that the USSR was also doing, they would refrain for commenting right away. Instead, they would use them to portray Western hypocrisy. For example, if the US attacked the USSR for putting dissidents in psychiatric institutions, they would respond that those attacks were coming from a country doing psychiatric experiments on their own population. Without such provocation, the Soviets would refrain from using "the dirt" directly, as seems to be the case in this situation. Barring a few publications for domestic consumption that can be still found on the Russian internet, the Soviets were content to let the Western press smear their own governments. They had their own secrets to keep, and did not want to be a proverbial black pot.

I think such news would find a place in short column articles in newspapers and in cartoons on international topics along with themes of racism, war crimes and religious backwardness and in political monographs but otherwise were little discussed in depth.

On the other hand, in Soviet sci-fi a topic where the capitalist or fascist governments or companies covertly add various bio-active substances to the food of the subjects or personnel or prisoners for experiments was quite prominent. For instance, the "Clay God" novel by Anatoly Dneprov employs the topic.

So, I would say it was mostly perceived as "capitalist business as usual".

Here is a Soviet book from 1986 by N.N. Yakovlev "CIA against the USSR" that discusses the topic.

P.S. @rs.29 in his answer pointed out an important factor: the Soviet reaction would depend on whether the USSR was engaged in similar activity.

Regarding the topic at hand, the USSR was engaged in similar (but not exactly the same) actions.

First, as @rs.29 correctly pointed out, the USSR widely practiced putting the dissidents in mental institutions. In those institutions the medics could and did use various kinds of highly-active substances on the inmates.

Could they use something entirely new to test? Well, I think to test something entirely new they would rather choose usual psychos first, not the politicals, and even for that possibly there were legal regulations, that is the medicines should be tested on volunteers first, but you know, it is easy to get consent from an unfree inmate or their custodians.

The second thing is that the USSR widely practiced testing various damaging factors (such as radiation) on unfree military personnel.

Whether such tests would require consent probably would depend on whether the damage inflicted on health exceeded some permitted limits. The limits for exposure of military personnel were very generous, basically assuming a nuclear war circumstances.

The personnel would be informed that they were participating in the test, and the measures to limit the exposure to some max permitted dosage would be taken but otherwise the soldiers would be simply ordered to go into radioactive area (with intended protection equipment).