Ice Beam is far from garbage. While it's no Avalanche it's usually tied with Blizzard for performance on any ice type that can learn both. Ice Beam makes Mewtwo the strongest ice type attacker currently in the game. Mamoswine is likely to oust it, assuming Niantic doesn't conspire against the brown woolly.
Thunderbolt puts Mewtwo as the 3rd best electric attacker behind Raikou and TS Zapdos. While that makes it currently better than all non-legend electric attackers that could change with Gen 4's release.
Flamethrower is the Hyper Beam of the new moves. It's better than other fire types in a generalist sense but when weather and specialist factors play in it takes a back seat to the usual suspects.
I'd like to stick an Ice Beam one on my ice type roster for variety's sake. Hitting hard while lacking the typical ice type weaknesses is pretty dope.