RPi 3B onboard WiFi chipset BCM43438 wireless LAN , with only 2.4 GHz support and Bluetooth 4.0 Low Energy (BLE)?
If so, it should work. If this will not work, please open a github ticket to get more investigation .
If it is a external USB WiFi stick, we need more information (chipset, USB PID, VID, REV, …)
This is the last RPI v.15x Debian Jessie image. No support, no fixes and updates by DietPi any more, only security updates by Debian via apt-get until mid 2020. Sorry.
For RPi? No.
But I personally don’t think RPi Debian Stretch will cause your Wifi problem.
Where is as well DietPi_v6.0_RPi-ARMv6-Jessie.img, but a little bit outdated.
It will install a lot of updates and patches on the first run. And we will not build a new updated basic image any more.
If you need a newer one, please build your own, see @MichaIng nice distribution. ( thanks for this and the great and easy working script! 
)
Why where is a DietPi_v6.0_RPi-ARMv6-Jessie.img ?
DietPi RPi Stretch build start about one ago. RPi Stretch status
But most people don’t notice, read, or follow this up.
Where are as well some “special” RPi builds, e.g. for AMIBerry or Allo, who take a little bit more time, to get it up and running with Debian Stretch as base.
At the moment @Fourdee work on a AMIBerry image to get it up and running under RPi Stretch. When DietPi RPi Jessie will be history.