The excellent Primer under the Sickert tab on this website gave one caveat and I quote: "I think at best we can say that Cornwell has found some very interesting connections between Sickert and some of the Ripper correspondence, which is certainly worthy of further investigation."
One can rarely prove a negative, so it will probably never be proven 100% that Sickert absolutely could not have been the Ripper, any more than Cornwell had any chance of proving 100% that he was. But he does seem to have said and done things that were bound to point some level of suspicion in his direction and that, if it had occurred today in the age of "political correctness," would probably have caused quite a scandal. If he could have seen into the future, I suppose it's open to speculation whether he would have thought better of his actions or if he would have enjoyed the resultant sensationalism and been of the belief that "any publicity is good publicity."
As for Cornwell, I think it's unfair to call her a "psychomaniac." She had very successful and respectable careers in both forensics work and writing crime fiction before the Ripper got into her blood (put there not originally by her own speculations but by Scotland Yard's John Grieve). Basically, I think her insistence on Sickert being the Ripper is no different from that of any other proponent of a favorite suspect. She just had the money and the tools to make her insistence very, very loud, and for her sake I really wish she didn't react with such dismissive disdain for anyone that disagrees with her. Being more open to healthy debate would have helped her a lot.
Finally though, I hope it might be possible to discuss Sickert as a Ripper suspect without even bringing Cornwell into it, because she was not the first to suggest it.
I think she put up a good arguement but too much of it is based on assumptions and possibilitys.
I think she genuienly believes she's right, however the way she draws conclusions she would have managed to convince herself whichever suspect she had chosen.