Re: The question of "Why would anyone particularly object to all this?"
Theme weddings aren't necessarily a bad thing. And even a kind of quirky theme wedding is fine.
Say you and your partner were super into astronomy. On your first date you really connected when you started discussing it, and a lot of your subsequent dates involved stargazing. It's a passion you both share, and as such you both wanted to commemorate that in the wedding's theme. It really wouldn't be difficult at all to come up with some truly lovely decorations based on outer space and the heavenly bodies.
Or say both of you are bakers. Again, this is a pretty easy sell in terms of color schemes for the decor, plus you could have a totally kickass cake AND a lot of other pastries for the guests to enjoy. It'd be fun, and it'd be a celebration of your passion in life.
Or maybe you two are dog trainers. You met through work, befriended a lot of coworkers and clients, and eventually went into business for yourselves. It's just been a huge part of both of your lives. It could be genuinely fun to ask your friends to bring their well-behaved pets to the ceremony, with the reception held at a huge park where the dogs could play with each other and with their masters.
I can see offbeat weddings and themes being just fine, if it relates to the two getting hitched in some genuinely significant, productive way. The electrical engineer wedding Kal-elk linked to is a good example of what I mean. Sure it's a little weird, but it adds a little personal flair to the event and isn't obstructive. And to that end, yes I can even accept the idea of a comic-, cartoon-, or even game-themed wedding. Hell, I can even accept subtle nerd stuff in a wedding between just two plain ordinary geeks, if it's not tacky or distracting. But that's not what's going on in this episode.
There's a world of difference between "our flower arrangements are based on the color schemes of our favorite superheroes" and "lol all the witnesses had to wear cheap plastic Power Ranger helmets and the priest was dressed as Galactus!!! XD SEE BECAUSE WE READ COMICS A LOT ROFL" The first example is just injecting a little bit of nerdery into your big day, while the latter is just jumping in front of folks and essentially yelling "GUYS I LOVE THIS POP CULTURE THING! I DUNNO IF YOU KNEW THAT BUT POP CULTURE!!!! NERD CRAP!!!! EXCELSIOR!!!!" while rubbing a copy of The Killing Joke in their faces and slobbering all over your Fantastic Four t-shirt.
Over the years we've looked at nerds who seriously have no engaging interests, and certainly no identity beyond "I like this TV show!" The ONLY thing they want to talk about is how much they enjoy easily digestible entertainment. It's totally fine to like that entertainment, of course, but to have that be the only thing you're ever willing to meditate on is plain unhealthy. Congratulations, you're a fan of a popular geek franchise, but why make it the only side to your personality? Why does everything have to revolve around the fact that you just really like Star Wars? Why do you have to introduce yourself as a Trekkie to everyone you meet, regardless of context or whether or not the other person would even know what the fuck you were talking about? Why must everything presented to you be compared to Kingdom Hearts?
That's what's going on in a lot of these weddings. It's less about celebrating your life together and your hopefully fulfilling future, and more about celebrating how much you love Ghost in the Shell. It's less about asking your family to share in an important milestone in your life, and more about writing big speeches about how very special of a snowflake you are and pointing out how you hot-glued Lego pieces to your dress because you're just that raaaandooooom~ It's less about writing meaningful vows, and more about laughing at grandma's confused look every time you shoehorned an internet meme into the ceremony.
Sure, these geeks are having fun. But see, that's because they find constantly pestering people about zombies and ren faire shit to be really fucking fun. I cringed when we read the Tropers' godawful stories about being annoying dorks, and I sure as hell cringed at these tryhards.