Mugabi was pretty good at 154 before he fought Hagler, in fact more than 1/2 of his fights in his prime were there, He was ranked there and moved up to fight Hagler. The one i gave myopinion on was the pre hagler Mugabli that one would have given Delahoya all he wanted early but still what have lost, The one who fought Duane Thomas and Norris would have been a much easier fight for Oscar.
Actually his best wins pre Hagler were against Middleweight (fringe) contenders. But even then he came across as limited. I know he was (thankfully) probably the last mainstream fighter being gimmicked as an African beast. But still, even when the Beast feasted on the Animal, it was brute power and aggression that won him the fight, IMO. You need a lot more than that to beat Oscar.
Hargrove,Gonzalez,Guiden and Green were all JMs his best wins outside of Fletcher and Parker who he beat at middle. I agree that he would have needed more to beat Oscar though.
I see your point about the Hagler fight being at middle, but I think Mugabi's peak was round that time and he was really a light middle when he fought Hagler, he very rarely came in at 160, even against Marvin.
But the problem Mugabi has, is his best performance was a defeat. After the Hagler fight, which I agree was while The Beast was still basically a 154lber, Mugabi had the boxing world at his feet. But when it came to prove himself worthy of that praise, he choose to quit against Thomas, and never again show anywhere near that level of performance he showed against Hagler.