I connected my MacBooks Pro 96W USB-C power adapter to the XPS 15 7590. I get a dialog with "Slow charger - To speed up charging, use charger that came with the device." and then warning saying "Weak charging state, please check power adapter".

Why am I getting this message, this is not an iPhone charger I am connecting to the laptop?

The Specs say for the Thunderbolt 3 (USB 3.1 Gen 2 Type-C) port with Power delivery
Supports USB 3.1 Gen 2, DisplayPort 1.2, Thunderbolt 3 and also enables you to connect to an external display using a display adapter. Provides data transfer rates up to 10 Gbps for USB 3.1 Gen 2 and up to 40 Gbps for Thunderbolt 3. Supports Power Delivery that enables two-way power supply between devices. Provides up to 5V/3A power output that enables faster charging.