Dell external monitor has blurry font
Unfortunately, that's normal. MacOS cannot display sharp UI when connected to a 23 Inch HD monitor. You have a built-in retina display which is 220 ppi. On the other hand, the monitor you are using is only 95 ppi. 

I had a similar experience with a HP ZR 24W, which is a 24 inch 1920x1200 display. I have tried this monitor using both HDMI and DisplayPort (thunderbolt), results were the same. Therefore, I don't believe your problem is related to the interface, since macbook pro 2015 can easily do 60hz 1920x1080 from its HDMI port. 

There are some ways to make text sharpness somewhat better by playing with text smoothing parameters (see this article), however you can only go so far with it. 

The real solution for you is to switch to a display which has more pixel density. One that will let MacOS to use the HiDPI retina resolutions. For that, a 24 inch or 27 inch 4K display will be an affordable choice. 

You can read my answer to a related question for more details. 

Hope that helps.