Sometimes it's just as simple as ghosting because even with a good idea, I'm not in the mood for it. It tends to be easier for me. I've tried the 'nice rejection' before. Not everyone is good with it, and that typically has led to people attempting to pressure me into agreeing with their ideas. Then it gets awkward/uncomfortable for me. So ghosting is preferred more often than replying; however, I can get a civil 'no' out when I feel confident a person will take said response well enough.
This. I ghost especially when I've interacted with them before, and I know it won't be taken well. Roleplaying is one of those few instances in life where I always tell people it's better to put yourself first even at the expense of """"common courtesy"""", mostly because it's not as simple as holding the door open for the next person or whatever. Sometimes there's some backlash that comes with it, and understandably, some people want to avoid this.
Also piggy-backing off the "too smutty" thing, I personally ignore messages where the first question directed to me about the story is about smut. That lets me know they are in it for one thing only, and while that works for others, it doesn't inspire me to reply. Besides, I am more than willing to be open and flexible about kinks if we've established rapport beforehand, making small talk and trading ideas back and forth.
I like to think of sending a RP request PM like making sale. In this case, you're selling yourself and the talent/skill you have to offer as a writer and roleplayer. If you're selling a car, you would never walk up to a new customer and go, "So do you want heated seats, a sunroof and Android Auto in the vehicle?" without ever introducing yourself first and showing what you have to offer, and what options might be a good match for them. Most car customers would be weirded out, and some might even be offended.
In a similar manner, I ignore messages that just go, "I saw your request thread for [plot idea] and I wanted to know if you were okay with furries, pegging, and choking while sucking dick?" While I might be okay with some of those, or even all of those, it doesn't catch my eye as something worthwhile to respond to. I prefer some rapport building and chatting first, but that's also just my personal opinion, which is neither correct or better than anyone else's.