The 'bitcoin is used for illegal activity' narrative is becoming less popular and using it as a means of attacking Bitcoin and other cryptocurrencies shows how little the government knows or is willing to know about decentralized currencies. Regardless of their position on Bitcoin, they do not have the authority to curtail the network, they could declare it illegal and limit exchanges from accepting transactions from certain regions, but cannot prevent onchain transactions as they have no control on the blockchain.
I usually do not expect approval from government agencies as bitcoin contradicts the existing financial system, their attacks shows it is functioning the way it was designed to.
It's a tactic used by the government in order to scare away people from Bitcoin. No matter what they'll try to do, they'll fail in the long run. That's thanks to the decentralized and open source nature of Bitcoin. If the government wanted to succeed, they would've taken action within Bitcoin's early days. Now Bitcoin is too big to fail as it's widely distributed worldwide. The network is extremely healthy with a vast number of miners, and node operators supporting it every step of the way. There's practically nothing the government can do to put an end to crypto/Blockchain tech.
Nonetheless, Yellen's comments about Bitcoin are not reason of concern among many crypto enthusiasts. As long as Bitcoin remains decentralized and censorship-resistant, there should be nothing to worry about. Centralized exchanges will be the ones mostly affected by fierce regulations by the government. The rest will carry on as usual. People will ultimately buy/sell Bitcoin via decentralized exchanges or in-person if centralized exchanges become too restrictive. Transactions performed on-chain will be difficult (if not impossible) to block by the government. Just my thoughts