Mutually Assured Destruction is really a development of ICBMs and especially SLBMs. The Soviet Union at this time had very few ICBMs: 20. The USA had far more and dominated by every other metric as well: IRBMs, MRBMs, and even the total number of nuclear warheads. Lacking the ability to make a second strike, 20 ICBMs is hardly enough for the Soviet Union to maintain any sort of second-strike capacity, Cuba presented an opportunity for shorter-range weapons to be useful. Yet it was not MAD-inducing. MAD is founded on the idea that enough nuclear weapons will survive an initial attack to still obliterate the opponent. Why would the Soviet weapons in Cuba or the miniscule number of Soviet ICBMs in the USSR have any hope of surviving a first strike? The other delivery system would be conventional nuclear bombs. Yet that delivery system is the most imperfect of all. Not to mention the fact the airstrips and bomb-storage centers would also be prime targets in a first strike (hence why air-delivered nuclear weapons are considered to have a first-strike stance, they are destroyed too easily), the delivering aircraft would face traditional aircraft defense. This leaves the Soviets with few nuclear weapons capable of engaging in an assured second-strike as the US had. MAD therefore had no effect and the Soviets would instead be incentivized to strike first, provided they had enough missiles within range. The limited number of USSR ICBMs meant full destruction of the US could not be assured, either in first strike or second strike, but having shorter-range (often considered tactical nukes) IRBMs and MRBMs in Cuba would mean the USSR had at least one way to definitively threaten the US with total annihilation: first strike. With the Soviets lacking second-strike capability, MAD had no role and instead missiles in Cuba increased the threat of nuclear war as it gave the USSR the power to and the incentive to strike first.
Hans M. Kristensen and Robert S. Norris, “Global nuclear weapons inventories, 1945-2013”.
Bruce J. Allyn, James G. Blight and David A. Welch, "Essence of Revision: Moscow, Havana, and the Cuban Missile Crisis".