Why does my friend want my maid of honor to look bad?

It's going to be a long 5 months with this one.

I agree with you she wants the MOH to look bad.She sounds petty and jealous.  But there's a lot wrong here beyond that. 

On the rest, etiquette does NOT say the MOH is responsible for the shower.  Showers aren't mandatory by any means, and anyone who want to offer one to the bride is free to do so.  She should check with the MOH just out of courtesy, of course.  Second, when you say you want to help with the shower, this is a bad look.  Because showers are one of the very few "gift mandatory" events, you should be completely removed from it.  The only role you'd play is to give names & contact info when needed.  Third, I've never heard anyone say showers are supposed to be a surprise.  The hostess can't organize it if she doesn't know for sure the bride will be there. Finally, you can provide some input into the bachelorette, but you're too involved when you say you'll plan it yourself.  Your role is to wait to see if/when someone offers to do this.

This chat may not have been a good idea.  Other than talking about the dress they'll wear, I'm not sure what there is to chat about. Your girls might want to set up their own without you for these other events.

Ignore her.

The group chat pertaining to the Bridal Shower and Bachlorette party should traditionally not include the bride. But all the " rules "are rapidly being changed these days. People seem to pick and choose what traditions to follow, which is their perogative.

There does seem to be some resentment or other negative feelings by the bridesmaid who keeps criticizing the MOH. Either ignore her or ask her outright "Do you have a problem with " x" being chosen as my MOH, as you are showing animosity toward her." 

Do you think she is jealous that she's not your MOH?

Maybe you should have a good talk with her about all this so it stops.

She obviously needs a course in Basic Keep My Mouth Shut 101 (especially to the bride).  As in the movie, you have a 100%, prime time, jealous, vindictive, petty, bm who is very POD because she didn't get to be MOH or has some mental issue against the MOH. IGNORE HER IF POSSIBLE, IF NOT She should not be annoying you, the BRIDE, with her petty poop. How to put her in her place, maybe don't take her calls? or when she does call, say 'oh, don't worry, Mary Anne (MOH) has that covered, she is doing such a great job. Well, gotta run, things to do and places to be, bye'. Don't stop for a breath, keep talking as to not give her a place to jump in.  Next time, "Mary Anne can handle that, give her a call (prep Mary Anne beforehand) that, she has been an angel to lean on. So, give her a call,love ya. Bye'----------------The other option is to confront her, gently tell her that all of these comments are nerve-wracking and everyone seems to have a different opinion* AND EVERYONE WANTS TO GIBE Tthat I don't really have time to deal with and if there is anything else that she thinks is wrong, just call Mary Anne, and/or**add if you wish---- she IS the MOH, we talk daily so she's got it covered". Not knowing your friend,1) she could blow up and back out; but, I don't think you are that lucky. 2) Be on your guard, if she has been thwarted several times she may have a full head of steam built up/not to scare you/ so she at the wedding could try to cause a bit of trouble or tension. HA!!! BRIEF BRIDAL PARTY TO WATCH OUT FOR HER AND TO "head her off at the pass" Remember---even if you are completely blindsided, just smile and say, "really, oh well" and remove yourself from her presence as soon as possible. IF and I say IF she does say anything you already know it will be embellished, told in a way to be unfavorable, etc. probably a lie so if you can just file this mess in file 13.  If not you or MOH, you preferably (I would) tell her you appreciate her offer to help; but, that you have everything covered and it's just the way you want it so there's no need to call about anything anymore. Also, contrare to belief it is not the MOH's place to pay or arrange for bachelorette, shower usually does; but, not a have to.

Sounds like you've got someone here who fancies herself an expert on all things wedding just because she once had one. Unless you're willing to damage the friendship you'll just nod and let her talk. This is like the person who believes they're a cardiologist because they had childhood heart murmur. 

Perhaps she's jealous she's not the maid of honour, so is creating drama / issues where there literally doesn't need to be any. 

just cut her out of your life and get on with it. 

Smash this while you can. 

I would tell your BM to please stop telling you and your MOH what you all should or should not be doing.  The wedding is some months off and there is plenty of time.  If necessary tell her you appreciate her caring but your wedding doesn’t need to like hers,and you can handle the planning, thanks,

No the bridal showers are not a surprise party. 

NO you are wrong, the bridal shower is NOT the MOH responsibility!!   They can, if they want to, throw a shower for you, but they are under NO obligation to do so. 

My best friend threw a bridal shower for my daughter. And the grooms aunts threw a second shower. 

No the BM should not of mentioned the shower at all. Unless she wants to throw you a shower and needed a list of guest.  She should not be telling you or the MOH that they should doing the shower. When someone plans a shower for you the BM can get an invitation.

Personally I would tell the BM to please only post positive  remarks.  

You don’t need anyone dissing the other women in the wedding party 

I think you are onto something with this. If this bridesmaid had only one, small issue with your Maid of Honor then that would be understandable. However, from what you have described in your question, this bridesmaid seems to have numerous issues with her. I would suspect that she is jealous that she did not get asked to be Maid of Honor and is therefore looking to expose her as being incompetent. I would not be getting this bridesmaid to message your Maid of Honor because she will only cause a conflict. Remember, she craves the position for herself so she is going to be a difficult as possible. My advice would be to tell this bridesmaid to back off.