First let me say the wild Un popular thing ,not every person our mom and dad "plays with" are going to be more popular than a can of paint at the store.
This is a fact of life , it seems to run in families that we are supposed to get along fabulously with new spouses old spouses etc.. not the way the actual world works.

Did you ever stop and think , there's a part of your mom possibly making up for some perceived "personal failing" in prior relationship and this man was beneficiary of that ?

Sure as someone above pointed out she was doing the whole proper Christian thing , but in my experience it's not usually that simple , your mom's behaviour could very well be something from her formative years , playing out , without a first hand intimate knowledge of you and her I couldn't say for sure.

In any case it's not hypocrite material in my hand to try and offer solace and support to a grieving parent or friend , you didn't have to love him or even like him , to stand in solidarity with your mom, grief is part of the human condition ,at sometime we all grieve , just as death comes to collect all of us sooner or later ,so to shall grief .

Being daughter family enough to accept the guy wasn't your cup of tea ,but still offering that olive branch is being human, if a batter relationship matures great , if not great also ,if we go in eyes open with low expectations either way ,any hurt can wash over us any healing can adhere .

Choose the high road of being human , putting aside our own burdens and baggage to help a "fellow traveler" ( i am sorry that sounds more Roman gypsyish than English ,i blame it on my roots) you dont have to come out ahead , to come out ahead ?
Mom will appreciate at some point in her mortal coil that you "helped" , you were the daughter most parents want us to be , at her side even if the fellow may not have answered any need or desire in you .

It's not always easy to play human and humane when your talking family expectations , family roles or family dynamics. But do as youd want someone to do for you .i hope this gives you clarity ,to makevthe decision that is right for you .