Dragon Pulse is essentially equal to Ice Beam against Arcanine -- STAB + NVE = no STAB + normal damage, it's the same multiplier in Pokémon Go.
So for the most common attacker, dragon pulse is ok for a defending charge move. But for pretty much anything else (besides Vape) you're gonna want Ice Beam / Blizzard. Plus, dragon pulse is terrible as an attacking move, so there's less versatility.
Overall, Ice Shard / Dragon Pulse is a decent moveset for defense but far from desirable. Lapras' strength is his typing and bulk, so really any moveset has at least some value.