Font display issue with 2018 Macbook Pro on External Monitor (2016 MBP is fine)
From the photo you posted it looks like the Dell is running wrong (low) resolution. Check that.

Also
Check what Profiles is used for the Dell.

In Color Sync Utility, in the Utility Folder.

In there you can run verify and repair function.



and verify what profile is used for the Dell.



Some background:

Apple's ColorSync Utility is built in to OS X. ColorSync profiles are used in image-rendering devices so colors will match across different devices. For instance, if you have two monitors attached to your system that are of different makes or models, then it is likely their pixel response ranges are slightly different. As a result, reds may be more vivid on one monitor than on another one, or the blues may be deeper. These variations can be allowed for by creating a color profile for each device, so when image data is passed between them they will display it accurately.

In my case, I do not have Dell to show you its profile.

Some more reading material.