Dave Mustaine tends to be full of himself, but his soundbite "bass is one step up from the kazoo" was obviously ironic and also eight years ago.
Like many tropes, "bass is easy", does have a grain of truth to it. Compared to drums, guitar and keyboards, bass offers two major advantages:
- It is initially a forgiving instrument for a beginner, once you master pushing down one string without too much buzz and hitting it with your other hand, you already have a bass tone that can be used somewhere.
- You're not the focus of attention, people - audience or band members - don't really care what you do and how you do it. No one watches for the mistakes of the bassist and most people don't even notice if the bassist makes a mistake because bass is harder to hear for the unaccustomed ear than guitar, drums or keyboards. Most people can't even discern whether a bass player is any good unless the band stops playing and lets him/her do some fretboard acrobatics. People can then see whether he/she is good.
Name me one band whose career was held back by having a not good enough bass player. I can't think of one. Alec Jon Such was deemed as not good enough by his band mates and fired eventually, but I have no recollection of enraged Bon Jovi fans leaving stadiums disappointed about his abilities (according to Richie Sambora, he hit a lot of bum notes live, but who would hear those other than his band mates?).
So, yes, bass is initially easier (mastering it is another matter) and its role in popular music allows you to stay under the radar without anyone pointing the finger at you and proclaiming "the bass needs to do better". And I don't believe that a single AC/DC, Rolling Stones, Boston, Judas Priest, Eagles, U2, Coldplay or Pink Floyd album was ever bought by the public because of anything the bassists playing there did or did not do. Still, those bands did alright - and so did their bassists.