We haven’t seen an economic slowdown caused by a virus / natural calamity during an election year. So it’s not comparable to past economic slowdowns. It’s a virus produced by nature, occurring first in a market in China. It affected the whole world. It’s not something he could’ve averted in any way.
I don’t think he’ll get the blame. Polls on this issue show that Republicans approve of the way he is handling it while Democrats disapprove (just as expected). So politically, not much here.
Independents are definitely going to be the deciding factor in this. If they don't blame Trump for the economy taking, he wins reelection. If they do then it's pretty much over for him.