Not hugely, it would appear to me. There doesn't seem to be much justification for rounding up all the eccentrics, as is so often proposed after mass shootings.
The numerator of psycho killers like Jared Loughner doesn't appear to be large, and it may be shrinking due to better medications and the like. This story is big news for a bunch of reasons (e.g., the press wants to use it to launch Obama's re-election campaign the way Timothy McVeigh launched Clinton's re-election in 1995), one of which is that it's a man-bites-dog story: Members of Congress and judges don't get shot very often at all. On the other hand, the denominator of people who suffer major mental problems at some point in their lives is very large.
As a small child, I recall seeing a black comedian on TV around 1967 or so telling a joke about how scientists say that 1 out of every 4 people are crazy... so think of 3 of your friends. They seem okay, right? So ... Congrats! (I can't remember who the comedian was ... perhaps it was Dick Gregory. God only knows what this fraction is for stand-ups.) I remember being shocked at the time by the 1 out of 4 number.
I'm not anymore.
I'm not being snarky. I've personally known a huge number of people who have gone through at least one major mental health problem. It's not like one out of four people are crazy at present. But that one out of four people will go through at least one sizable mental health episode during their lifetimes seems utterly plausible to me by now. The amount of suffering and sadness in this world caused by mental illness is immense. What's more surprising to me now, and thus gladdening, is how many of these people have gotten better: some with medicine, some with therapy, some who knows how or why, but they've done it.
P.S. Jerry Pournelle writes:
"Allowing the non-violent madmen to live among us is a price of liberty; and allowing physicians and police to lock people away because they are mad is conceding a power to the authorities that often proves unwise, and sometimes is simply an adjunct to tyranny."