I think that there are two issues here that you need to consider;
1. Airframe ice will form when flying in actual icing conditions. That means in the case you describe you would have to be in the cloud unless it was raining and the OAT would have to be 0 or below (can happen a few deg above 0 so icing conditions are often defined as OAT less than +5 and visible moisture).
2. Ice will form best on objects with small radius facing into the airstream. That means in order of first to last to show icing;
OAT probe or similar (not on the C150 as it is in the air vent)
Strut
Tail
Wing
and some where down the list, the tyre.
So if you were below the cloud and it was not raining it is very unlikely that you would pick up any icing no matter how cold it is.
Having said all that, with the high moisture content of the air close to the cloud base, you could expect a lot of Carb icing at all power settings.
Regards,
DFC