There's a lot of TV shows out there, even in the 80s and 90s, and plenty of ways for celebrities to have their image and reputation bolstered. Ratings aren't reliable in trying to measure someone's notoriety.
Growing up in the late 80s/90s, and mostly outside of the US, I can't remember a time when I didn't know who Bob Ross was.
Did you grow up wealthy in the 80s? Most people didn’t have cable television back then, it was comparatively expensive and not available outside major metropolitan areas. Most people only had a half dozen TV channels or so, and sometimes Bob Ross was the only thing on TV worth watching. Everyone knew who he was.
We were not rich but had basic cable since 1979 or so. Maybe California was ahead on that front. My memory is that it only cost perhaps $10 a month in the 80s.
My experience was that accidentally tuning into Joy of Painting for about 45 seconds was enough to completely hook me (although I was not fully aware of this at the time).
Inside the US, never heard of him until later 2000s or so as well. And watched PBS at times.