I think its wrong to compare the Mini-14 with an AR-15 type of rifle. They are made for different purposes. Ruger calls it a "Ranch" rifle in that it's designed for taking cars of predators in that sort of environment, around a farmer's ranch getting rid of coyote's, wolves, varmints on either 4 or 2 legs.
The Mini operation is based on the M1 Garand, the AR is (was) all new in terms of operation and it's purpose (basic infantry rifle). The Ruger GB series of the Mini-14 was (and still is in some places) used by a number of police organizations, a large number of state prisons for their tower guards and there was even a European version that was tested as a infantry rifle.
The barrel of the Mini Ranch rifle is thinner than the AR barrel and accruacy does suffer with a high rate of fire. IMHO, that is one reason why the Mini is seen as inaccurate. A thin barrel and a high rate of fire is the big problem when it come to accuracy comparisons.
My Mini has a few mods like a muzzle brake, a small gas port bushing and a Hogue synthetic stock that holds it tighter. Those items alone enables me to shoot quarter-sized groups at 50yds on a rest with the factory peep sight. I use factory 55-gr ammo from either Winchester, Remington/UMC or S&B with similar results. Going out to 100 yards, it's 1.5" to 2" groups. For me that is perfectly fine for what the rifle is.
From what I have read, the new "Target Mini" is supposed to much more accurate out of the box compared to the Ranch Mini. A new bull barrel, new tooling, and most likely closer tolerances.
Having said all of that, I will be picking up an AR in the next month or so just to have one. Not that I think it's any "better" than my Mini, I just want another semi-auto .223 rifle.