Im pinging ~150ms on average.

I have a 100mbps internet plan. Would upgrading to a 300mbps internet plan improve my ping ?

How are you testing your ping?

Hopping into a game/server and /fps

Is there a better way?

Yeah, never test on a game.

Go here and run about 5 tests at various servers around you... http://www.speedtest.net/

I ping well to servers around me, but only ~150 in game on east server. Wondering if increasing my bandwidth would improve my ping in game.

It's probably the game server itself. With 100mb/s you should be fine on any game server.
If the game has various servers you can choose from, try another. It might just be the server you are on is seriously congested.

Diablo2

Maybe check Blizzard forums. Maybe something is going on, like server maintenance or something.

Do you live on the east half of the country? I live in MN, and my ping is around 40-50 on either server for D2.
Also, d2 is a 20 year old game with 20 year old servers. They are just not that good compared to modern hardware.

I live in Halifax, Nova Scotia.

I wonder if it's just purely location, or if I'll somehow be able to get a lower ping.