I'm no Corbyn fan but I don't know if it would be fair to pin the seat losses on him personally, especially given the revealing prediction that Labour stands to lose over 100 seats even if they match the Tories nationally in terms of vote share (and, admittedly by a slightly naive prediction, might need to win by over 5% of the national vote in order to make any gains at all). They just had that strong a result in 2012, in part because the Lib Dems got pummelled, that it would be difficult to match it. But even losing 200 seats is likely to leave the party something like 750 seats better off than the Tories, and that's assuming that all the seats Labour lost were Tory gains.
Essentially I reckon the result is going to look Bad for Labour almost whatever happens, but by the quirks of local election cycles. It would seem more appropriate to compare what happens in the coming local elections to, say, the 2015 Labour results nationally. Any signs of a vote share north of that result would appear to be in Corbyn's favour, ie that he is (slowly) bringing voters back to the fold after Ed Miliband's failures. If he does only match the 2015 result it looks pretty bad, though.
What I'll be most interested in is where the Lib Dem's remaining 400-odd seats are going. Maybe that will help to mitigate Labour's losses?