It depends on the mobile wallet you are actually making use of, some mobile wallet are very much decentralized and that way it would be very difficult for hackers to gain access to your wallet to steal coins, one of the mobile wallet I make use of I am keeping large amount of coins and never regretted doing so is coinomi, coinomi is a decentralized wallet that gives you access to your private key and store all the datas directly on the blockchain with a cache in your mobile phone making it difficult for any hacker to penetrate into the wallet.