I would contest the idea that Ichigo "sucks" at Kido, because he doesn't know any, and no one has ever bothered to teach him. There's a difference between being a terrible martial artist, and not being a martial artist.
To be terrible at it, you have to know some, but be bad at doing it. Ichigo is the "not a martial artist" in this particular scenario. He's not bad at it, because he doesn't know any of it. Being 'bad' at something requires a capacity in whatever the something is. Just a low quality capacity.
As for whether or not he would be any good with it, that depends entirely on how it works. Zangetsu is confirmed in manga canon to help Ichigo concentrate and compress his spiritual energy. Depending on the actual mechanics of Kido, that could be an incredible aid and boost, or it could do approximately jack squat.
And since we don't know how skilled Ichigo might actually be in it (as like I said, and you mentioned, he's never tried it or been taught it), it's hard to say, beyond factoring in outside things like Zangetsu's abilities.
However, Ichigo
does have highly refined supernatural abilities, which he basically came up with himself. Rukia outright says that it's mindblowing for someone who's never been trained a day as a Shinigami to use a technique that some Captains have trouble with.
Ichigo also managed to preform a Quincy exclusive technique just by hearing Uryu describe how it works once.
So going off of that, I'd say that Ichigo would probably be a fairly exceptional student if he was taught Kido, regardless of whether or not Zangetsu's abilities would help.
why does everyone seem to think that Ichigo is a complete dumbass? He isn't. He was ranked in his school. Obviously, he is a good learner at the very least if he can get a ranking.
Because people are used to dealing with "clever but stupid" shonen protagonists, and thus mentally lump Ichigo in with that group automatically.
The fact that Ichigo 'appears' to be stupid in-context isn't helping. By that, I mean he knows essencially nothing about Soul Society or the spiritual world in general, and has to ask all the 'obvious' questions.