To add insult to injury , Minato did not create the reaper death seal. The Uzumaki clan created it and they even created a mask that could summon the reaper as well. Minato is just the first character that we saw use it. Poor Minato if you really think about it. Minato kind of had a habbit of being outclassed. Think about it:
1. Minato took 3 years to develop rasengan and he never even succeeded in adding a nature to it. Yet Naruto learns it in a week , Boruto learns it in what I believe is even less time. What's more , Naruto quickly and easily derives a way to add a nature to it , and Boruto even adds a nature unconsciously! Jiraiya , Kakashi and Konohamaru also learn rasengan in much less time than it took Minato. The final nail in the coffin is when filler takes the glory of inventing rasengan away from Minato and gives it to Ashura (and has Ashura be better at it than Minato).
2. Minato was by far surpassed by Naruto in the realm of senjutsu. Madara brushed him off like an ant.
3. FTG used to be thought to have been invented by Minato , but then the war came and the credit was given to Tobirama.
4. Naruto was much better at using KCM and BM than Minato.
5. Reaper death seal turned out to not have been invented by Minato.
Poor Minato. At least his FTG still is said to surpass Tobirama's , so that's good.