Italy's contributions were indeed decisive in the Entente's victory in WWI, namely in exacerbating Austria-Hungary's collapse by continuing to bleed the country of manpower in the Alps after Austro-Hungarian successes in closing the Eastern and Serbian fronts.

By the time Italy entered the war in May of 1915, Austria-Hungary found itself in the unfortunate position of having to fight a 3-front war against Italy in Alps mountains, Russia in the East, aided by Germany, and Serbia and later Romania in the Balkans, aided by Bulgaria. By the end of 1917, after initial setbacks in 1914 and 15, Austria-Hungary along with Germany was victorious in occupying Serbia, forcing Romania to accept an armistice, and forcing Russia out of the war in the wake of the Russian Revolution, securing Austria-Hungary's borders to focus on their single remaining front in Italy.

Italy continuing to remain in the war, especially after Italian defeat in the Battle of Caporetto, prevented Austro-Hungarian troops from having the freedom to move and bolster Germany's position on the Western Front. An Italian blockade of Austria-Hungary's coast along the Adriatic was also essential in denying food and supplies from entering the country, forcing Austria-Hungary into a food shortage crisis along with Germany that increasingly outraged the civilian public. This discontent among civilians in Austria-Hungary was especially dangerous as ethnic Czechs, Slovenes, Bosnians, Slovaks, Croats began voicing their outrage in the form of calls for self-governance or outright independence from Austria-Hungary.

Italian victories in 1918 along with continued economic decline and food shortages caused by the continuation of the war proved to be the final straw for many of these minorities. With continued Entente pressure on Austria-Hungary by Italy along with a string of defeats at the hands of the Italians at Monte Grappa and the Piave River throughout the summer of 1918, the Austro-Hungarian monarchy finally collapsed under pressure in the Autumn of that year, with many minority soldiers deserting or simply refusing orders from their Austrian and Hungarian officers, the First Czech republic declaring independence in October, and finally, a knockout blow by the Italian army against the Austro-Hungarians at Vittorio Veneto, with Austria-Hungary losing over 500,000 soldiers in a period of 10 days, the monarchy collapsed and was forced to sign for peace shortly before Germany's collapse.

In short, Austria-Hungary had many shortcomings when it declared war on Serbia, but managed to secure 2 of its 3 frontlines by the beginning of 1918. Italian persistence to continue the war and continue exerting pressure on the Austrian-Hungarian economy and army eventually led to a complete collapse of the country due to an Italian blockade of incoming goods as well as Italian victories that outright defeated the Austro-Hungarian military by October of 1918.

If you want to read more, check out The Italian Army and the First World War by John Gooch

Edit: typos