Alexandra was more of a symptom than the cause of the downfall. Seriously. It goes way too far to believe that the Russian people gave up on the Imperial system because of anything Alexandra did or did not do.
I agree that Alexandra was in good measure a symptom more than a cause of what was wrong with Russian autocracy by the late 19th century. However, I would not absolve her entirely of fault in bringing down the monarchy.
It really hinges on the question of whether, despite all the stresses in the system arising from industrialization, emancipation without land reform, and related tensions, autocracy could have somehow weathered the storm of WWI had Nicholas not made some incredibly unfortunate decisions after 1905 and especially from 1915 onward.
If one is of the view -- and I'm not entirely sure that I am -- that another revolution could have been avoided had a different tsar been on the throne after 1905, then I think Alexandra does bear some blame for bringing down the monarchy. She was resentful of Witte and implacably hostile to Stolypin, both of whom might have had a chance to stabilize Russia had Nicholas given them more unstinting support. And it was her hostility to both that influenced in some measure Nicholas' shabby dealing with them. (Remember that she even had Stolypin spied upon.) And it was certainly her encouragement that Nicholas refuse all pleas for ministries responsible to the Duma, her insistence that Nicholas reject his counselors' advice and replace his uncle as Supreme Commander of the Military, and the almost unbelievable spinning jenny of mediocre or downright absurd ministerial appointments that she and Rasputin worked to bring about once Nicholas was out of St. Petersburg that had a lot to do with destroying any remaining fragile chances of holding the government together as Russian military reverses mounted.
As each of the pillars of monarchical support collapsed, Alexandra's handprints could be found somewhere on the side of the pillar. She might not have had the strength to push it over alone, but she did her share of the heaving, albeit unwittingly.
The senior nobility and even some Romanovs became so disillusioned with her that they were willing to undermine the tsar's moral authority and the dynasty's reputation to get at her. (Not only did crabby old Aunt Miechen run her mouth all over Europe, but Marie Feodorovna made astonishingly candid and disparaging comments on both Nicholas and Alexandra to foreign emissaries. And GD Dmitri and Russia's richest heir actually turned to murder.)
The senior military commanders, having seen the tsar's performance as Supreme Commander and his malfeasance in civil government as the ministerial system became a widespread joke in the hands of his wife, came to view abandoning the tsar as the only means of avoiding military defeat.
The dominant voices in the Duma -- none of whom sought the overthrow of the monarchy -- became utterly disillusioned of being able to work with Nicholas while he listened to his wife on ministerial appointments.
Nicholas' own Council of Ministers finally despaired in the final days of getting him to accept any advice that did not come from or through his wife. I have wondered if they did not ultimately give up on the monarchy mainly to get Alexandra and Rasputin out of their hair.
Finally, even the Orthodox Church -- and this is far too little discussed by historians -- lost faith in the monarchy due to the rise of Rasputin.
Now one could argue that a stronger Nicholas would have alleviated the damage that could be done by a bafflingly clueless Alexandra, but the fact is that they worked off of each other -- not in all ways and not totally . . . but certainly enough for her to bear some blame for the water the regime took on and that ultimately sunk the ship of state.