Even with such a small primary mirror, New Horizons has a number of advantages. First, Ultima Thule is larger than Manhattan. Not exactly large, but certainly bigger than the Empire State Building.
Second, the mount is incredibly stable. There are basically no vibrations to speak of. As an amateur astronomer, I can tell you that the mount is one of the most important pieces of equipment in astrophotography. A mediocre telescope on a good mount will give good images. A good telescope on a mediocre mount will give mediocre images.
Third, the mirror itself is much better designed and manufactured than the average amateur astronomer's scope. Certainly better than any of mine.
Fourth, the sensor is cooled to very low temperatures, so it is way less noisy than the average consumer sensor.
Fifth, there is no atmosphere in the way. This is a huge advantage.
When all is said and done, LORRI will have a resolution on Ultima Thule's surface of about 60 feet. Certainly no Google Earth, but plenty good enough for the science objects of the flyby.
My point remains though, and it was just a general metaphor about how astounding the stuff they are trying to do is from an imaging perspective.
https://youtu.be/ymJRlUQfPfQ?t=586
EDIT: The full download of the gigabytes of data from the flyby will take 20 months to download.
The best shots won't be for a while, but hopefully the team received some informative images today or tonight and will be able to share them at tomorrow's press conference. As she notes, there is a chance that the first downloads will miss Ultima Thule entirely. The encounter was just that difficult.
As for LORRI and Ralph, can't we call them both "primary?" Only Ralph produces color images, so most color shots of Pluto are Ralph's (https://solarsystem.nasa.gov/resources/639/the-frozen-canyon...).
LORRI has higher resolution, which is why some of the most stunning shots of Pluto are black and white (https://solarsystem.nasa.gov/resources/652/plutos-majestic-m...).
Some images were produced by "painting" Ralph's colors onto LORRI's high resolution features (https://solarsystem.nasa.gov/resources/699/pluto-dazzles-in-...).
As a computer vision/imaging person, that's a pretty far look for a 75mm (~3 inch) mirror. It's like trying to take a picture of the Empire State building in NYC from Las Vegas with a tripod telescope.