To be fair, you can say a lot of blowers is an ATG if you only take particular aspects of their career.
Imran is an ATG, no question about it. Whether he is top 5/in that absolute top echelon is where people are often split. Some make allowances for peculiarities in records, some don't; and that affects their rating.
Mediocre is a pretty strong word I won't go as far as to say that really. Its just that his batting alone was good enough his bowling just came as a bonus pretty much.
How would you describe it? Statistically, It is not even as good as the average bowler's average or SR of his era. It is not even as good as the average spinner's average or SR, let alone fast bowler, of his era. So it is not average and a bit worse than that. In my books you can call that mediocre.
I agree with your perception and I've argued that people were less statistically inclined then. Or maybe better said; they valued different stats.
I remember when the commentary of peers and experts about his bowling popped up it usually revolved around bowling average and wickets-per-match. On these basis, Sobers was probably not that bad most of the time, and as you say was a bonus. However, SR is really important and he was not a fine striker of the ball. He was also expensive especially in the initial period where he was a spinner.
He was a part-time spinner, part-time pacer; bowling full-time overs. It's one of the great mysteries to me how someone with such a bowling record could be lauded so highly as an all-rounder. If batting makes up for bowling deficiency it'd be safe to say that Bradman is an awesome all-rounder.
IMO unless you're a minnow side or one whose bowling is weak you simply do not bowl Sobers. In an all-time side picking him as an all-rounder is a bit of a joke IMO. There is such a drop in quality from the specialists to him that it is a waste to give him overs. That's why to me being a bowling all-rounder gives you even more; for all bowlers have to bat but not all batsmen have to bowl.