If I may throw in my two cents here...
...I personally wouldn't say there is a lack of creativity, but a lack of determination to try something new. Often times, we tend to stick with something we're already familiar with, rather than try something that isn't in our area of expertise. Its the logic of "if it ain't broke, don't fix it".
I would say, with a lot of people in the MUGEN communities, especially a lot of the big name creators, the tend to stick with what they know, rather than what they've never done before, hence why... a lot of stuff, tends to never exist or never come to light.
Even though there is opportunity to try something new, and many of these people have the skill to do so, the problem is, whatever it is that is new that they are trying, they wouldn't really know how to do.
Cause here's the thing, people either go for:
If they go for style, then often times, whatever they are making, has absolutely 0 accuracy to the original source material, thus they might as well have just stuck to something previous of that same style without having to tread too far from the accuracy.
If they go for accuracy, then these characters often fall short of what the "expectations" of the community around them wants. Don't get me wrong, I'm sure many of us have seen a game or two and were like "man, I would love to see X character inside MUGEN", but then if we think about it logically... most characters, with accuracy in mind, wouldn't last very long or be very enjoyable, unless they were specifically designed to be in a screenpack for other characters similar to them.
Remember, a good number of people who have MUGEN characters, tend to mix and match. Very few of us actually play the "full games" as they are intended to be, and often times, prefer to just take everything out of these screenpacks and full games, and shove it into our own.
Time and time again, I have only downloaded a full game, just to get all the characters from it, and nothing else. I can probably say, with a high certainty, that most people who play MUGEN or even watch MUGEN, have a roster which is a mish mash of everything they could find.
The only times we actually see something from games that are either not known, or obscure or certainly from a less popular perspective, is when it is then made by someone who's pretty terrible at making characters/stages/etc.
So I wouldn't say that there is no creativity left in MUGEN, but that most people in MUGEN, don't feel comfortable or feel like putting in the effort into something that either wouldn't work out very well and get a lot of negative scrutiny for not being the next POTS-style edit or whatever, and instead, just stick to what they know and keep themselves from falling into the same category as some other creators who've tried something new, failed miserably, and then got lots and lots of backlash.