Electrum is best option for altcoin (which based on Bitcoin/Bitcoin fork) developer, many altcoin have their own version of Electrum.
Mycelium is also good option, even though i never see altcoin wallet based on it.
Mobile wallets for bitcoin-rpc is easy to create. Heck, you can even do it with a wordpress website.
It's possible, but bitcoin-rpc isn't designed for such task on large scale.