Here is my ranking of jobs you can be incompetent at in the band and still be in the band.
1. Singer. We have all seen it, even major league bands have questionable singers. It's why Autotune exists.
2. Bassist.

I might have even put this as number one, but I know my audience. Lots of bands with coat hangers for bassists. Few can distinguish it in a mix aside from other practitioners. Quality in this role is almost unnecessary.

3. Keyboards. Generally most keyboardists in bands are pad holders, B3 drawbar manipulator/leslie circumnavigators or piano chart readers. No one buys a performance keyboard without some skillset. So incompetent keyboardists are a rare breed, but they usually come in the flavor of band member S.O. with a side of tambourine.
4. Guitarist. As long as he can widdly widdly, everything will be fine. I've made it work with just about anything.
5. Drummer. If your drummer sucks. Your band sucks. There is no other way about it. I have carried many a 'drummer' through the night. But it was never pretty, nor fun.*
Funny though the singer can be questionable, but not awful. The line is somewhere around Karaoke. It's that difference between DLR or Van Hagar. Hagar is top tier rock singing royalty. DLR is an entertainer who also happens to be a very capable 'Vocalist'. I'll forgive a lot in singers if they have a personality voice. Ultimately I have never really had to, I've been spoiled in that I have only ever worked with top notch singer/entertainer/front people, or played in instrumental bands. I have however been subjected to more than a handful of Satans minions who claim to be drummers, no time, no groove, no dynamics, but man they got fills and stick twirling tricks.
*Imagine if you will a scenario. A band on a gig. The bassist, unwilling to participate in the drinking reverie, stay sober. The bar, packed on a Saturday night. The second set began with a Reggae medley that is on a sequence. Bassist notices that the drummer starts in a strange way and looks over to see with

horror that the drummer is not wearing the headphones with the click, and he is quite drunk

... Bassist. Armed with an SVT and an 810, pins the bass knob, plays hard articulate time with the rhythm track on the sequence that he can hear through the front of house while the drummer played 'Jazz'. I got a gold star for that piece of wizardry.