The article “Rising Sun” posted is informative [Italian Army in WW-II] and it does show the Italian mechanized forces were quite good and I do recall that histories of the Africa Korps also make mention of this. That the Italian mechanized units worked well along side the German mechanized Divisions. Lack of mechanization in campaigns , where ones enemies are mechanized, puts one at a decided disadvantage in such battles/campaigns. Essentially you must cede initiative to your adversary. However mechanized units consume twice as much supplies compared to simple leg-mobile infantry units of corresponding size and this became a critical problem for the Axis.
With reference to the naval situation, I was reading O’Hara’s “Battle for the Middle Sea” and it looks like the Italian navy did very well in its primary mission of supplying the forces in North Africa. It seems that despite occasional incidents like the RN attack on “Beta convoy”, most such convoy attacks were failures. Overall something like 90% of the supplies and troops, the Axis sent, reached their destination. In its worse incarnation only in late 1942 did the supply fall to maybe 3/4 reaching destination. Infact a primary factor in the Axis defeat looks like the arrival of the Americans in late 1942.It was just too much for the Axis to handle.
Further if you look at the combined aero-naval campaign the Axis waged against the Allied forces in the Med, it was quite effective through late 1942. Through out 1940 the Italians convoyed 5 times as much supply as the Allies did and as long as the RM functioned, Allied convoys from Gibraltar through Alexandria were impossible. In Malta during 1942 only 1/2 the supplies sent to the Island actually arrived, forcing the inhabitants into starvation diets of 1200-1800 calories a day. At several points the Governor of Malta reported they were just a couple of months away from having to capitulate, because of this situation.