I think memory is cheating re: rubbish monsters in classic Who: Off the top of my head Erato, Mandrils, Plasmotrons (and that's without having to think hard)

There are also plenty of decent stories with really naff monsters, if not quite at the level of Erato et. al - e.g., the eponymous Loch Ness Monster. Being a bit forgiving about the monsters is a requirement to enjoy a lot of classic Who though, of course, it maybe isn't something that should be asked of viewers of modern Who.

The Abzorbaloff is definitely no worse than the Slitheen though, I'll give you that that is a pretty low bar.