I know it’s a old threat. When you upscale a DVD, then you have VobSubs, low resolution image based subtitles and this looks really ugly.
How you can do it better
Use open source tool subtitle edit drag you’re mkv file directly into the program, choose you’re subtitle from the list and wait until it’s importet.
Under “OCR Method” choose “Tesseract 5.3.3” (or the newest one). In the Field “Language” click on “…” and choose the dictionary language that matches you’re selected subtitle and click “download”
uncheck “prompt for unknown words” then click on “start OCR”, when it’s done you can export it as SRT-File and mux it to you’re video, for example with free tool MKVToolnix
