That could be for a variety of reasons.It always amuses me when the True Fans get on their soapbox with the line "this isn't Star Trek, it's generic science fiction." I guarantee, ask the vast majority of Joes and Janes on the Street what "generic science fiction" is they're almost certainly going to say Star Trek. Unless they say Star Wars. My point is, despite the True Fans' protestations to the contrary, according to fifty percent of Normies out there, Star Trek is generic science fiction.
If some fans criticize current Star Trek for being too generic, maybe these hypothetical Joes and Janes are also calling Star Trek "generic science fiction" because they've only been exposed to the JJ Abrams films, and DISC and PIC.
Or maybe they consider it generic science fiction only because the franchise itself is so iconic and ubiquitous now. Most people I know don't even watch Star Trek or like it, but would probably cite it if asked "What's science fiction."
I would be surprised considering that many, many episodes of TOS and TNG don't feel like generic sci-fi, but either way that's something else entirely. There's different levels to this. People have different interpretations of what's underground, mainstream, indie, etc.Even within the sci-fi community, I'm sure many would associate Star Trek as generic sci-fi.
I think much of the criticism is that some new Star Trek properties are leaning more towards the action element of film and television.I mean, generic sci-fi is basically about an ill-defined military-like service that denies it's military wearing uniforms that are either multi-coloured and/or jumpsuits in ships with improbable FTL capabilities having run-ins with human-like "aliens" who of course speak English and planets with a hidden mystery. Not only does that sum up Star Trek, it also sums up a good many other sci-fi properties too.
Most people I know wouldn't admit to liking science fiction, don't watch Star Trek, and don't know who Isaac Asimov is or Harlan Ellison, but they loved Aliens and Terminator.
The most notable change in this action side seemed to be TWOK. TMP was more about exploration and wonder, but it didn't make a big enough profit. So they went more mainstream, making it more action influenced and it was a hit.
TNG on the other hand wasn't very action-oriented as a whole when it started, so no surprise that it wasn't an immediate hit. Picard being the anti-Kirk and all.
For me the TNG films definitely tried to be more mainstream, generic sci-fi. FIRST CONTACT was heavily influenced by Aliens and maybe a bit by Terminator. They even use something more similar to assault rifles or pulse rifles instead of traditional phasers. "Lock and load" was from Insurrection I think.