How big a problem are pickpockets in Prague?

Is Prague bad?
Prague is no worse than any touristy city, better than some I would say.
As always, if it happens to you, the impact can be huge.
The risk is the same as at home, but when in a foreign country you will need to sort things out much faster and in a place where you do not speak the local language.

What to do to diminish risks?
Be prepared, do not keep your important papers and cards where thieves can easily reach it. And best not keep them together.
Keep your passport in a pouch under your clothing, possibly with one of the bank or credit cards and most of your money.
Have some of your money, with a card that is not working, in a purse or wallet which you have near at hand and will not mind losing. Keep the rest of the money and your cards in a purse, wallet or pocket of your bag which is not as easy to get into. Copy the photos of your camera or phone onto your computer (if you bring one) or the cloud or whatever once a day if you take many photos or whenever you have photos you would hate to lose, have your phone backed-up before you leave home so in case something does happen, you do not lose all your information/numbers. If you sit down, like for a drink, do not put your phone on the table or next to you when sitting on a bench, but hold it in your hand till you put it away safe, and keep an eye on your bag. Best also have a strap of your bag around your leg if on the ground or around your arm if high up enough.

Not just your phone?
The same goes for your wallet, of course, maybe even more so.
Never put your money and papers down where someone else can pick them up. Best do never get out your main money and papers unless you are somewhere safe where people can not just walk past and grab it.

More extreme robberies?
Muggings are very rare, you run the least risks if you do not stray into dark alleys at night.
When going out do not accept drinks from a stranger, keep your drinks in your hands and in view, the risks of someone dropping something in it are low, but the risk exists in every country.

Get knowledge!
Most important, read about the city, know what you want to do, where you want to go and so on.
That will help you concentrate and keep an eye out for people behaving strange near you.

It can still happen, be prepared!
But even with the best preparations you might become a victim of a pick-pocket, do not worry about it too much and do not feel a victim unless you got hurt. Have copies of your important papers with someone you trust and have a mail ready with the information should you need it. It does not hurt to have a list of what to do when your papers are lost, contact phone numbers for your bank and where to go for help, (like contact details for the embassy) which are often in travel guidebooks.

Do not worry too much, you will have a great trip.

I've visited Prague twice in the past five years and, as mentioned in another answer on this page, I didn't feel at any greater risk of being robbed in Prague than in other tourist destinations throughout Europe.

Rick Steves has written extensively (in his guidebooks and online) about how to avoid pickpockets when visiting Europe, and one of his top recommendations is to wear a money belt.

It helps to understand the types of locations where pickpockets prefer to operate. You are more likely to be pick-pocketed at popular attractions where large groups of distracted tourists are already bumping into each other. The Prague astronomical clock is an ideal example of a high-risk spot.

Pickpockets also target passengers on public transit, especially during peak commute periods when the stations and tram/metro cars are full. During those times, I used Uber to get around, avoiding not just the pickpockets but also Prague's notoriously scammy taxi drivers.

A mobile phone with international data coverage (T-Mobile offers some good options) is a compact, multi-purpose tool that can really enrich your travel experience. Most older smartphones can help with navigation, translate signs and menus, and may even take reasonably sharp pictures if the subject is well-lit and not moving too fast. Cell phones attract thieves, though, so keep your main phone back at the hotel, locked inside your luggage or a room safe, and only go sightseeing with a phone that you wouldn't mind losing. Before your trip, perform a factory reset on your sightseeing phone to erase any personal information that could be used to steal your identity.

While all the other answers are good, I would like to make the comparison in a more objective way. According to numbeo's safety and crime index, which is crowdsourced by contributors, Prague is a very safe city to visit, with the following score:

• Level of crime: Low
• Worries home broken and things stolen: Low
• Worries being mugged or robbed: Low
• Safety walking alone during daylight: Very High
• Safety walking alone during night: High

Also, Prague is the 20th safest cities among 82 major or reported cities in Europe.

Another statistics is international homicide rate, which puts Czech Republic as one of the safest country in the world, at 0.7 per 100,000 people.

That being said, as with all the cities in travel, you should care about whatever you should.

So I'm certain that Prague should be considered safe to visit.