I only got to watch some episodes as a teen, & I loved them. As an adult, I got to watch almost all the episodes. Most are dumb, some are fun, and a handful are utterly hilarious! With that ratio of good/lame episodes, I can understand people not liking it if they only watched a few times. Personally, I wouldn't mind a few more episodes.
Heavy metal music not being very popular wouldn't be a problem for the show. A lot of the time they were getting mad & frustrated because there wasn't anything cool on TV. Ripping on lame videos was what they did most of the time anyway.
My favorites were:
When Beavis misunderstood a lot of stuff, wasn't feeling well, and thought he wasn't feeling well because he was pregnant.
Beavis & Butthead forget how to urinate.
Beavis gets a bloody nose, & Butthead tries to help by doing all the wrong things. (That was a very bloody episode.)
They try to have fun by doing "Ding Dong Ditch," but can't figure out how to do it right.