Is it wrong to laugh?

I haven't seen this movie yet but I know exactly what you mean...I still laugh at parts of "Suicide Club" and I've owned it for over a year and a half now. In fact, when I first watched "Suicide Club"...and for a long time after...I was completely convinced it was a dark comedy and there was no telling me otherwise. Actually, I considered it the best dark comedy I have ever seen because it managed to be much more dark than any other movie I'd seen and still remain funny at the same time (to me). Genesis' song, the woman happily cutting off her fingers with that cheery song playing, and the massive spray of blood when the students jump off the school...hilarious I think. Don't worry...you're not alone in finding Sono's films funny.

It’s ironic that in this age of “tolerance” we are the most afraid of offending people.

Haven't seen this one yet but Suicide Club also made me laugh. While disturbing it used some awesome (extremely) black humor to address serious matters.

Did you ever notice that people who believe in creationism look realy un-evolved? - Bill Hicks

I think Sion Sono has a VERY dark sense of humor. Like, just take the famous opening to Suicide Club. He sarcastically put that accordion song in it to have SOME humor.

As for Strange Circus, I laughed at the abrupt cut (no pun intended) to scissoring.

He does it on purpose. So it's disturbing, but not to the point where it's REALLY disturbing.

Mmm Exactly what scenes made you laugh? Just curious. I dont recall laughin when I watched Strange Circus, but I did find some scenes amusing.

Yep it's his style, and I laughed at some of it too. People expecting coherence and "normality" may sadly not be able to enjoy this films. Here's what he says about it in an interview with Offscreen.com:

Offscreen: Again I think the film is very sophisticated in this way, but there are parts of the film that are comic and parts that are serious. I thought the first part of the story up until the part where the girl is to be guillotined, because she wants to die, was very serious. And if it had been funny, it might have been a problem but other parts of the film were funny and it was OK. Did you know which sequences you wanted as comic and which as serious? Was it difficult to switch from comic to serious?

Sion Sono: I think film has the power to make audiences realize that if you are alive you will have painful experiences, and you’ll get hurt. Showing the audience that you can laugh about it helps audiences laugh at their own pain or sorrow as well. I don’t want a film to go too deeply into terrible experiences or a person’s sorrow, but a shifting and balancing between them.

Read the whole interview here (it doesn't provide much insight as to what Strange Circus is about, but it's quite interesting nonetheless): http://www.offscreen.com/biblio/phile/essays/sion_sono/

It depends which parts you laughed at. There are some scenes in movies that are serious/horror that are funny.

Which parts were they?

I can't say I recall laughing out loud, but In retrospect there were some genuinely humorous scenes. (There were probably more few horrific ones that I snickered at, specifically when Yûji was beating his dismembered father on the bed.)

I only saw this movie recently in a Horror Festival in Lisbon, but I can certainly say I had a smile on my face for a good part of the duration.
It is brilliantly twisted in black humour in several parts, which only made that much disturbing when I noticed the smile being out of order in moments of pure suffering for the characters.
It's all part of the game, the calm pleasure of twisted humour just before it shows you are (partly) just as delighted in pain as anyone else.