Why did Italian artillery perform so badly during WWII?

You can't just look at the specs of a weapon, and draw your conclusions from that. The artillery may be good, but it was only part of a long chain. That entire chain was rusty at best.

I don't know about the artillery, but captains in the infantry were responsible for training their men. Many if not most were not good in it or even trained to do that.

Artillery needs a lot of supplies. Not just ammunition, but also spare parts, lubricants, food (for the crew), fuel and a lot more. Especially in logistics the Italian army had a major problem, acerbated by the British naval blockade in the Mediterranean. One of the reasons why the Africa Corps and the Italian army were defeated is that 70-80% of their supplies were sunk or destroyed before it could reach them.

The higher ups in the Italian army got their position based on nepotism, be it a noble rank or a high rank in the fascist party. Nobody denies Italian soldier weren't brave. They were very badly led. Literally, lions let by donkeys.

Relative obsolescence and lack of mobility

75/32 Mod. 1937 was apparently never used in North Africa. This was relatively modern gun, with dual purpose (indirect artillery and anti-tank role, similar to Soviet ZiS-3). However, production was slow, with only 5 pieces completed in 1937-39, up to 30 in 1940-41, 44 in 1942, and 98 in first half of 1943 before the capitulation. After the capitulation Germans captured and used 48 examples. 75/32 was mainly used on Eastern Front with Italian 8th Army where they acquitted themselves relatively well, even against Soviet T-34 tanks. It should be noted that in USSR guns of this type were used in motorized units which increased their efficacy.

75/18 Mod. 1934/1935 was a mountain gun-howitzer with relatively low muzzle velocity, usable to target attacking infantry formations, perhaps even to suppress enemy infantry in dugouts and fortifications, but ineffective against vehicles, even not armored ones, due to curved trajectory of the shell. Later in the war self-propelled Semovente da 75/18 was armed with this gun, and effective HEAT shells were developed, but main problem of low muzzle velocity impeding targeting remained. Overall, even this gun was not produced and used in much quantity, especially in North Africa. Italian sources mention employment in Greece (96 pieces), Russia and Tunisia.

75/27 Mod. 1911 and Mod. 1906 was actually main gun employed in North Africa, for example during Operation Compass, alongside its brethren Böhler 8 cm Vz. 1905 and Cannone da 65/17 modello 13. Again, these were WW1 era guns, usable against infantry on the open, much less against fortifications and even less against vehicles. In North Africa especially, Italians lacked primary movers in their infantry divisions - sometimes these guns would be towed by trucks to a position and left there practically immobile, while trucks went to haul another cargo. This allowed British to simply bypass them, and to focus their strength at weak points.

47/32 Mod. 1935, not an artillery piece per se, but anti-tank gun. Relatively ubiquitous in all Italian formations during the war, also used in Italian tanks in modified war. Reasonably effective up to 1941-42, especially against British Cruiser tanks, but practically ineffective against heavier Matilda II tanks. Later in the war they were facing increasingly armored opponents (M3 Grant/Lee, M4 Sherman, Valentine, T-34, KV-1 ...) and became totally obsolete. As with other Italians pieces, when left without organic transport often bypassed and isolated.

90/53 Mod. 1939 was an AA gun with dual role of being a very good anti-tank weapon, similar to German 8.8 cm Flak. Total production number is estimated as 539 before the capitulation, unfortunately most of these were employed in their primary role as AA guns in territorial defense. Italian sources mention 240 for territorial AA defense (without transport assigned) in 1942, while Italian army had 80. In North Africa they were often mounted on Lancia 3Ro trucks, thus creating some kind of mobile tank destroyers. Flaws of this weapon were its large size and lack of armor, thus limiting its use to fire from ambush positions and greater distances (out of effective range of tank guns). In any case compared to other Italian pieces relatively decent and modern weapon, but fielded in small quantities.

Finally, heavy artillery should be mentioned, and typical example of this would be Ansaldo 149/40 Mod. 1935. Heavy gun, with long range, employed both in North Africa and Russia. In North Africa, according to Italian sources, they fought with some distinction, from siege of Tobruk to el Alamein and back to Tunisia. Used to bombard distant objectives, and sometimes engaging in artillery duels. As relatively expensive pieces they were motorized and would retreat ahead of other units. Main flaw was simply their numbers, and Italian inability to support larger quantity in the field.