Maud Pie was written as a "I like rocks" character probably since season 5. As a result, almost everything she says is a running joke related to rocks. That explains why Mudbriar was written the way he was.
And about the "Maud was my waifu" argument, as soon as someone says that, he loses me. You don't have to be in love with a character to justify why you don't like another character. You can say whatever you want about Flash Sentry, but I think it's weirder that someone like Twilight falls in love with him despite not having interesting conversations. And nobody pointed that out, they were like "nah, Flash Sentry sucks because he stole my waifu".
C'mon, we're better than that.