1) Severe tornadoes haven't gotten any more frequent since the 1950s. If anything there's been a mild decline:
One caveat: It's possible that tornadoes were under-reported in those earlier decades, before Doppler radar coverage became pervasive, although the NCDC is fairly confident in its data set for tornados stronger than F3.
2) Tornadoes do seem to be getting slightly costlier in recent decades. Here's a chart of tornado losses, in inflation-adjusted dollars. Note the big spikes in the 1960s and 1970s and then the slight upward tick in the last decade or so (this chart also excludes 2011, which would be off-the-charts expensive here):
3) But those increased losses are largely due to more people living in tornado areas. Now look at this chart from Kevin Simmons, Daniel Sutter and Roger Pielke, Jr.:
They adjust the tornado losses to account for the growth in U.S. population, the increase in housing stock, and the fact that we're richer than we used to be — a standard way of "normalizing" disaster trends. Looked at this way, tornado damages have actually decreased slightly since the 1950s, albeit with a huge uptick in 2011, a year that saw six EF5 tornadoes in Missouri, Oklahoma, Alabama, and Mississippi.
In other words, the tornadoes themselves aren't getting more destructive — we're just putting more stuff in the way to destroy. Here's how Bill Hooke of the American Meteorological Society once put it: "[A]s we continue to concentrate our population, the chances for a truly catastrophic tornado event inexorably mount."
4) Fortunately, U.S. tornado deaths are declining over time. This chart comes from Harold Brooks of NOAA:
Even though more and more Americans are living in areas where tornadoes roam, the number of tornado deaths per capita is declining sharply over time — possibly due to improved forecasting and warning systems.
If you want to see total number of fatalities (that is, not adjusted for population growth), Alexis Madrigal passes along this chart, which shows some of the deadliest tornadoes in U.S. history. Note that the 2011 outbreak in the Southeast and Missouri really stands out:
5) Scientists aren't sure whether tornadoes will become more or less frequent in the future. See this story by Andrew Freedman or this old post by Andrew Revkin on what scientists do and don't know about the connection between global warming and tornadoes. Both pieces are interesting throughout, but here's the short version: There's not a lot of great data to work with. No one can really say one way or the other at this point.
Freedman sums up the debate nicely: "While a warmer climate is likely to feature more opportunities for thunderstorms to form, studies also show a lessening of atmospheric wind shear, which would suggest a decrease in the potential for tornadoes to form. How these two trends play out — one increasing the odds of tornadoes, the other reducing them — is a subject of active scientific research."