I think people are just mad that IM3 made so much money and was so well received by critics and audiences. Anyone that says it's "TRASH" is deluding themselves. It was a solid movie that WENT AWAY from the norm. The easy thing would have been to do another cookie cutter hero vs big bad guy. Sure, did I want Mandarin? Yah; but the twist was funny and original and DARING. MCU took a chance and it payed off. Maybe not to certain sectors of comic fanboys; but the general audiences enjoyed it. Iron Man 3 is the 5th highest earning comic book movie of all time for a reason. Sure, it was riding high on Avengers hype; but it also was well received and people really enjoyed the humor.
I have issues with it but I still consider it a good film and CERTAINLY better than Iron Man 2 without a doubt.
But yah, Iron Man 3 is better reviewed by both critics AND audiences so if it's "TRASH" then BvS is definitely trash as well. I wouldn't go so far to say that; I thought BvS was mediocre.