I would actually ask, how much did the minimum weight rule change in 1995 change anyway? Did it put anyone into a notably better or worse light? Of course as we remember it didn't help Mansell at all...
The main one that comes to mind is Katayama, who was very light. Then again it is argued he lost competitiveness due to cancer, so not a clear-cut case either.
Katayama does come to mind, but it's not something that I've analysed. It could be worth looking into though.
So that I don't do two posts in a row - I think the two main reasons why Mansell isn't seen as a proper great by a lot of people are that 1) he took a while to reach his potential, being overshadowed by De Angelis at Lotus and then Rosberg at Williams until he seemed to find his feet at the end of 1985, and 2) He only won one championship, whereas the other top drivers of the late 80s (Prost, Senna, Piquet) all won at least three.
But I don't think that performances earlier in a driver's career should necessarily influence how their whole career is seen. I think it's about having a decent period of high performance regardless of what happens before or after that period. And arguably he was just unlucky to win one championship. Most of the championships won by the other three were close, and they could have won more or less if chance had gone another way. We all know about what happened in 1986, but in 1987 Mansell was the dominant driver but suffered far worse reliability than Piquet. Also, in 1991 after Senna won the first four races, Mansell actually closed to within 8 points after Germany but suffered from poor reliability and a horrific pitstop in Portugal. So it could have been four.
By the way, despite what I have said, I would still rate Prost overall above him as he was more consistent over his whole career and was rarely out of contention for the title in the whole period from 1981 to 1990. But I'm not sure I see any particular reason to say that Piquet was better than Mansell other than the number of titles, and Piquet was arguably on the luckier side in the championships where he was in contention. When they were team-mates, I would give Mansell the nod regardless of Piquet arguably suffering after his Imola crash. I think Mansell was overall the more effective driver in 1986 anyway.