In Italy the victory over the Austrians in the Great War is always celebrated with great style

Is it though? Around here there was an exposition and a public conference, I think, of fairly questionable scholarly value...

The history (and historiography) of the Great War in Italy has a rather complicated legacy to deal with, and has often been unable to do so properly. Career diplomat and delegation attache at the Paris Conference, Luigi Aldrovandi-Marescotti, had this to say in the introduction to his memoirs (Guerra diplomatica, 1936):

The Italian war of 1915-18 […] didn't enjoy, until after the fair revision of fascism, its rightful position of prominence, neither during the period of actual fight, nor during the period of Bolshevik-like Italy.

Which is sort of the central issue. Fascist historiography did take control of the Great War and position it as a foundation moment in the formation of the Italian nation, of a national conscience (which arguably it was, for good and bad), of those healthy portions of the country which rejected and fought back against the Bolshevik threat - as the beginning of the Fascist revolutionary restoration-renovation, culminating in the March on Rome of October 1922. First the Risorgimento with the national unification - then the victorious Great War which completed the unification process and provided the material and spiritual instruments for the affirmation of a new generation shaped by the trenches - and last the Fascist revolution which brought this generation to power.

The Great War - from the intervention, through the crisis of Caporetto, up to the "greatest victory won by any army ever" in Vittorio Veneto - was therefore the decisive moment for the italian nation to resume her advancement (an "Italy on the march" - to quote a famous expression of historian Gioacchino Volpe).

This sort of legacy - which didn't affect the historiographical production only in its ideological terms, but also in practical ones (Mussolini did personally discourage supposedly "revisionist" inquiries, even if coming with the broadly hagiographic approach of contemporary military history) - has certainly discouraged approaches to the Great War as a "national experience", as a moment of formation of the Italian nation.

So that, still in 1976, historian G. Rochat could observe how the Italian historiography of the Great War was conspicuously lacking in any "revision" of the Fascist narrative - dominated either by technically accurate military works, or by retellings of the patriotic glories sung during the Twenty Years, destined to the exhibitions of military antiquities, or on the other hand by a complete rejection of the whole affair (with its corollary of incompetent generals, criminal politicians and indifferent kings).

It is certainly too simplistic to claim that the hegemony of Fascism over the Great War has led many to toss the baby with the bath water (as various works existed with the explicit intent to reclaim the Great War as a liberal, or even popular experience - see for instance those of A. Omodeo, E. Lussu, G. Minzoni); but even those efforts, aimed at a vindication of the Great War from Fascism and from the fascist deformation, suffer from their polemical nature and from the need to constantly confront the Fascist presence, forced somehow to play on someone else's ground.

As for the central point of your question: of course it did! Which is not the same as to claim that the Entente would have lost without the Italian intervention - outside of the counterfactual nature of the question, it is really much better to look at the Great War as it unfolded, rather than under the lense of some hypotetical scenario.

During the years from May 1915 to November 1919 Italy employed around 5.8 millon men in the Army, achieving (albeit in a rather cumbersome fashion) a degree of social and economical mobilization unprecedented, and unrepeated, in Italian history. While the Austrians - in part thanks to their defensive advantages and in part thanks to the Italian offensive struggles - managed to hold their front with a smaller amount of troops, the Italian presence still diverted a significant force that could have been employed elsewhere. Similarly, the Italian fleet, while it failed to knock out the Austro-German forces in the Adriatic and Mediterranean sea, did at least free some French and British units much needed in the time of submarine war.

Now, as you can see from both these examples, the Italian presence did help - but, one could argue (and more than a few did argue), helped much less than it had been expected when, arguing in favor of the last concessions to be included in the Treaty of London, Lord E. Grey explained to Russian Foreign Minister Sazonov how the Italian intervention could be “the turning point of the war”. Now, he was trying to make a point, but he expected something more from the Italians - and it's hard to judge impassionately the struggles of someone else when you have to face immense challenges of your own - especially given how the Italians appeared to have leveraged their intervention to gain as much as possible from their allies. The Russian Ambassador to London, Benckendorff observed immediately after the signing that

The imperfections of the treaty from the point of view of the Slavs were clear and evident to anyone. […] [But] Every military authority was adamant in proclaiming that the alliance was urgent. To obtain more from Italy would have required a significant time. To negotiate during peacetime for an eventual war is an entirely different thing than negotiating during the critical stage of a war with a power that is talking with the two sides and to which the circumstances give a weight disproportionate to its intrinsic strength.

Similar sentiments were expressed by French President R. Poincare in his memoirs:

Indeed, a bit excessive those first [...] Italian demands. Perhaps our neighbors have obtained some conditional promise from the Austrians that they want to compare to the benefits they can gain from the belligerents. The fact is that, before they enter the war, they ask from us and clearly, besides an access point to Ethiopia […] the control of the Adriatic and of Eastern Mediterranean. That's a lot […] Delcasse examined the territorial revendications of Italy […] He believes, like I do, that we should look at them with a favorable attitude, but there are three points that pose serious concerns: the Italian occupation of Dalmatia, the neutralization of the Slavic territories given to Montenegro and Serbia, the recreation of a Muslim Albanian State.

And the same "excessive" nature of the Italian demands can be traced in the communications of Benckendorff, to Sazonov, where he declared that

[British and French] fully realize the inconveniences you foresee, but military considerations are now overwhelming […] The Italian role can be of primary importance from a military point of view and her attitude determine that of the other [potential belligerent]

The latter would be Romania, whose Prime Minister, Bratianu had solicited a (never finalized) agreement for a simultaneous Italian and Romanian intervention (the Italians very likely didn't trust the Romanian government given its German ties, nor wanted to antagonize the Hungarian portion of the Dual Monarchy) - yet the Entente was at the time hoping for Italy to be the first piece of a domino of Romania, Greece and Bulgaria.

As you can see, for the Allies, given their expectations and the almost four years of war from the Italian intervention to the Armistice, it was easy to regard the choices made in bringing Italy into the war as somewhat of an impulse purchase - regardless of the fact that italy, they agreed, had done something to help, it had been a rather expensive something.

Nor did the Italian tendency to exaggerate the danger of their position after the Russian collapse help much in endearing the Allies - the famous episode of the alleged 21 German divisions at Caporetto (it was six of them, as Foch and Lloyd George both pointed out) was only one of many - but overall the entire Italian diplomatic conduct, their reluctance to engage in propaganda with their Allies, their not unjustified but poorly hidden sense of resentment for the supposed disregard of the British and French, the growing tension at the Peace Conference, favored the development of a climate where a "negative" opinion of the Italian war appeared both convenient and easily justifiable.

Again following A.-M. already in his final report composed after his return from the Allied mission to Russia (February 1917):

During our long journey, there was a constant feeling that France and Great Britain had a much stronger voice and direction power than Italy does […] Whether because of certain deficiencies or inadequate application on our part, or because of the more energetic efforts of the others, there is no doubt that the labors and the immense sacrifices made by Italy do not appear to be held in proper esteem. We find ourselves in a state of isolation in too many different matters: a very dangerous position, the closer we get to the decisive stages of the war [...]

What am I missing? I lived in Italy for most of my life and I don't recall ever seeing ot hearing about any celebration at all for "victory against Austria in WWI". The incredibly informative answer by u/Klesk_vs_Xaero (thank you!) suggests a likely explaination for that.

The closest thing I can think of is that, in almost every city and town, no matter how small, if you look for it you will find a list of local people who died in the conflict, typically engraved in marble, sometimes in brass. In smaller towns, it can be one list for the entire town under a monument. In bigger cities, there are usually several lists, say, one for people of a suburb, in a church, one list of carabinieri in their headquarters, one inside an university, listing killed students of that uni, and so on. All of them date back to the beginning of the two fascism decades, or just before then (in any case, between the two world wars).

My question to the OP is how did he/she get the impression that anything relayed to WWI is celebrated in modern Italy. Just curious.