Is there a Portal setting to increase the resolution of portal web maps/apps. The same vector features on ArcGIS Pro Desktop are significantly higher resolution than the same on web maps/apps. An example is a tilted line in a web map shows noticeable zig zags. This image is stretched for discussion purposes, but clearly noticeable when not stretched also.
I've noticed this, and it seemed to be tied to how those layers were published. Do you see this consistently across all layers, or just certain types? In our case, one of our computers had anti-aliasing turned off, and layers published from it retained that setting in how they displayed.
Not a direct answer to your question, but ArcGIS Pro is using the power of your workstation to generate those images from the data it receives. There is often significant power available for that single user.
When you're browser requests a map image layer across the internet, then that request is coming from an ArcGIS Server. That is a shared resource, and very often has a similar level of CPU/Memory as an ArcGIS Pro machine, but serving many users.
If you double the DPI of the image generated, then you quadruple the effort the server has to undertake to generate that image. That's why, typically, the DPI is set at 72 or 96 DPI so that you get a map with some performance, but the quality isn't as good as Pro.
Generally, people that want "Pro" level quality for their cartography will install ArcGIS Pro rather than using web-based tools.