Regardless of what his army of fan boys think, Klopp is still a relatively unproven manager. He's managed in just one country, and had proper success in just two seasons. When the bar is winning league titles and becoming a European force, promotion hardly cuts it.
He didn't keep Mainz up for years, he kept them up for two seasons, before getting relegated in the third. He then quit after just one season back in the second tier because they didn't go straight back up. It's also a complete lie to suggest that he built them up from nothing. They had a good crack at promotion not too long before he took charge, and were a pretty well established second tier side for a long time before they gained their first promotion, having been there consistently since 1990. Mainz were then promoted the season after Klopp left, and have remained in the top flight ever since. In large part due to the efforts of his replacement at Dortmund.
Mainz weren't a nothing, minnow of a club when he became their manager, they were an established second tier side. It also isn't completely unheard of for a team to earn promotion to the Bundesliga for the first time and remain there for longer than three seasons. Augsburg have done exactly that, and they were in the 4th tier when Klopp took charge of Mainz.
His apparent Lazarus-esque revival of Dortmund is hugely overstated as well. Yes they'd finished 13th the season before he got the job, but a trajectory of 13th, 6th, 5th then 1st isn't exactly unheard of in Germany. Wolfsburg had gone from 15th in 06-07, to 5th in 07-08, to winning the league in 08-09, and that was under Felix Magath. Werder Bremen managed something similar, finish 13th in 98-99, then winning the league in 03-04, with a few 9th to 6th placed finishes in between, all under Thomas Schaaf. Then there's Stuttgart that went from 9th to 1st in consecutive seasons under Armin Veh.
No one thought Magath, Schaaf or Veh were the second coming of Christ when they did something similar, nor did they think Klaus Toppmoller when he took an unfancied Bayer Leverkusen to the Champions League Final in 2002. The reason Klopp is so highly regarded is because Dortmund played an exciting brand of football, a brand not too dissimilar to the style Rodgers played in 2013/14, and because he's a bit of a nutter.