Downloading illegally-distributed files is illegal in some jurisdictions (but not too many, mostly only uploading - i.e. distribution - is illegal). That's true. So, use of Popcorn Time (or clones) for illegal things could be, indeed, illegal.

Also, in some jurisdictions linking to illegally-distributed content is illegal. Now, that's exactly what Popcorn Time does. So the point is valid - the very software (and its distribution) could be ruled to be illegal as well. Just as, say, Napster. It's just that Napster could be shut down for good and Popcorn Time couldn't - due to decentralized nature - but that's not a legality issue anymore.

However, what's very likely to be not illegal here is a development of the said software. There's very unlikely to be such a meta-law that forbids the very development of software that may result in linking to illegally-distributed materials. Sometimes there are attempts to apply a anti-malware laws, but that's quite a big stretch that works only in corrupt and/or ignorant courts (like some in Russia).