I'm sorry to say but yeah a person can say anothers opinions are irrelevant.
You can say my opinions are irrelevant, but what if I'm not arguing a matter of my own opinion? What if I say, "It's intellectually dishonest that Go-onger is about "fighting pollution" when the series is itself an ad for toys made with ecologically destructive manufacturing processes?"
This is a point of argument that would have basically nothing to do with my opinion of the series (which is "needed to be funnier"). To argue the point I raised, you'd need to be willing to discuss any one of several very objective issues.
1) Is it intrinsically hypocritical to use toyetic properties to promote ecological themes? How does Go-onger compare in that regard to, say, Captain Planet?
2) Are the Bandai manufacturing processes employed in making Go-onger toys particularly destructive to the environment?
3) Are the toys ecologically destructive? Can they be recycled? If sent to a landfill, would they be able to biodegrade?
4) Does using a conservation theme in an otherwise comedic and light-hearted children's show trivialize the seriousness of the issue?
Now, you could dismiss my concern as unimportant or maybe personally uninteresting-- but I don't think you could dismiss my point as basically not meaningful. Not without engaging with it and actually constructing a coherent counter-argument based on factual, falsifiable assertions.
Anyway, my feeling on criticizing a show is that you shouldn't do it in any of the show's main threads unless you're watching it regularly. It's just
rude. If you dropped a show at episode 4, nothing you say about it is likely to be all that interesting to someone who's on episode 37.
I think it's okay for criticism to come up in secondary threads about a series, particularly when it's part of a broader conversation topic ("I dislike use of two-episode arcs in Kamen Rider and this is why") or part of a post-series post-mortem on the show (like the post-series W threads).
So for instance the stuff I'm posting about Go-onger I would only post now in a thread like this, that's about looking back on Go-onger. Actually I find it pretty interesting to read why people actively liked the show, since I couldn't get anything out of it myself.