SMG's first 60 stars are easy.
The next 61 stars are a pain, especially the purple coins.

No, I don't think it's easy. I know I'm echoing others when I say this, but "beating" the game is easy (Bowser) but completing the game isn't. On 10 or so stars I know I've lost at least 10+ lives on (mostly the daredevil ones, weird changing gravity ones and of course the Luigi's Purple Coins).

Daredevil Golem was very, very hard. 20+ tries to beat.

It depends.

To beat the game minimum, nah, it's not that hard, but it's not super easy.

To get all 120 stars, there are like 30 really hard stars, I still haven't been able to beat 3 of them... 

It seems like later posters agree, 60 stars easy.  But 60 stars is like using warp zones and not playing through the "A" levels.  I think that the sixty stars is harder than using warp zones, or the whistle.  I don't think it was easier to get to 60 than on Mario 64.  I think I am just more used to 3D games now.

To get 60 stars, yes it is quite easy
To get 120 - not a chance
(I got 242)

I really liked SMG, but it was WAAAAAY to easy for my liking. I beat bowser and then wanted to get all the stars, but i just got bored because i didn't really feel challenged and just quit. The one level that i found to be quite challenging and VERY fun was the one right before the bowser fight, you know where your running inside the tube and giant bullets are being fired down it. I thought that was awesome and wish the whole game had that sort of difficulty.

 This, the game as whole is a nice chalenge, but only the minimum is an easy task...

Depends if you beat it or FULLY beat it.