Mahathir is the root cause of UMNO weakening. But he did make UMNO strong once during the past 16 years.
Statistics don't lie.
1999 General Election : Mahathir made UMNO lose a staggering 24% of their seas - down 22 seats from 93.
2004 General Election : Mahathir made UMNO strong by retiring from politics, no longer contesting and keeping quiet and letting his replacement do his work.
Immediately, UMNO won 53% more seats and jumped from 71 seats to 109 - in the best results ever for UMNO and BN.
2008 General Election : Mahathir made UMNO weak again by continuously attacking and treasonously attacking his own party over years - which made UMNO weak again by losing 38% seats.
In 2013 General Election : Najib won 9 more seats for UMNO - hence strengthening UMNO.. Mahathir was also relatively quiet this period.
Thus, this is indisputable proof where we can draw two conclusions:
1. Mahathir is the main cause of UMNO weakening.
2. If you want UMNO to be strong, ask Mahathir to shut-up.