Why not? If I'm of the opinion that the sound of the "mainstream" is uninteresting, unoriginal and mostly talentless, why shouldn't I say as much? This is a forum for discussing music. It would be a pretty dull place if everyone kept their opinions to themselves.
Being a mainstream act doesn't necessarily mean your music has no merit. Some bands become mainstream simply because they attract a wide audience; Dave Matthews, Pearl Jam, Muse, etc. However, if you sound like a mainstream act, by definition it means you're not very original. If you sound like a mainstream act, it probably means your music is devoid of any truly compelling musical elements.
When you speak about talent, you need to consider that it's largely subjective. So when you say they're talented musicians, compared to who? Clearly they're more talented than someone who doesn't play music. However, are they talented when compared to the bands regularly discussed on this forum (Circus Maximus, Pagan's Mind, Redemption, etc.)? No.... not in my opinion.
Why? I've never understood this mindset and I likely never will.
I love a lot of bands that people choose to take shots at; Manowar, Disturbed, Lamb of God. However, I could care less. The opinions of people, sitting at their keyboards, who I'll likely never meet, have no impact on my life or on my opinions of the music I choose to listen to. That anyone cares what some stranger thinks about their music choices is completely baffling to me.
Zod