After his failed attempts at painting and and architecture; Hitler fled to Munich in 1913 escaping the Austrian call to service. In which he then became an army courier and lance- corporal in the Bavarian army. Hitler wasn't the ideal soldier, not the athletic type and considered an "oddball, quirky and introverted". He manages to survive 4 years among the trenches; still as a courier. From what I remember he was wounded twice and awarded the Iron Cross 1st class. Ian Kershaw's "Hitler" quotes that he was unfit for promotion due to his lack of leadership abilities.

After the armistice is signed, he manages to squeeze out 2 more years on the army payroll, often considered his "Missing years". Later he is made an educational officer where he began lecturing at universities, where he found his calling as the prominent speech rabble rouser that history usually portrays Hitler as.

Apologies to the mods, because this won't comply with subreddit standards. But I think the subject is so infamous that there is the distinct possibility that all the sources we have on this question might be biased.

So pure speculation - but I'd like to point out that according to the man's own account, he thoroughly enjoyed and found himself in the camaraderie of Army life. In my opinion, it's very likely that makes him at least an adequate or satisfactory soldier. An army is a team. How you get along with your mates is hugely important. The measure of a good army is not in how many hulking terminator-sized sociopathic killers who shoot lasers from their eyes and fart fireballs out of their arses but can't get along with anybody you have in your army. It's in cohesion, discipline, camaraderie, the guy to your left being willing to jump on a grenade to save your life, and vice-versa.

So if he remembers his comrades and his experience alongside them fondly, I think that's a strong indication that he was at least adequate, and his mates thought he was adequate. Probably wasn't a Rambo (those guys tend to get killed off first in any war) but that's not necessarily the most important thing in determining who's a good solider and who's not.