Zimbabwe is a far better team than Afghanistan. The problem with the Afgan team is that they are highly inconsistent, I do not think it will be a problem for Zimbabwe to reach 345 runs. They have started well and they will easily win this test too.
I don't think that there is a lot of gap between Afghanistan and Zimbabwe. Even with their best players (Brendan Taylor, Sean Williams, Kyle Jarvis.etc), Zimbabwe would struggle against Afghanistan. If you are talking about Ireland, then I would agree. These two teams are ahead of Ireland, as the latter has been on a downslide ever since it got the test status. Ireland is overdependent on Paul Stirling (who is not getting any younger), while Zimbabwe and Afghanistan have somewhat balanced sides.