So the ～にすぎない grammar is used when you mean something doesn't get past some (high) level. Can I use it to mean something doesn't get too low, in a phrase such as 「自分の国の冬は、低温が15度に過ぎない。」? (meaning, the minimum temperature never goes under 15 degrees)
You seem to be asking whether or not
に過ぎない can be naturally used to express the lower bound of something. The answer to this is no, for multiple reasons.
First of all, when talking about exceeding/going past something specific, you want
を過ぎる, but even then
過ぎる means something closer to English's
go past than it does
exceed. You can see examples here, but it's most often used for passing by or through something, either temporally (such as passing your peak) or physically (such as passing a car). The closest example I can find to what you are trying to do is a webpage here that says
マイナス５度を過ぎると。。。, but I am fairly certain this only works because the negative number is in some sense growing in size - a little like English's
get past negative 5.
を過ぎる cannot generally be used to talk about something decreasing or being lower.
Second of all,
に過ぎない is a set phrase meaning something like
to be nothing more than. This is touched on briefly here and you can also see examples here, but the point is that
15度に過ぎない doesn't mean
doesn't go past 15 degrees, it means
a mere 15 degrees. Note that
過ぎる can (as you may know) also be attached to verbs and is used in several other set expressions.
All that said, for talking about a value that a temperature doesn't get higher than, I would in general use 超える which is a lot like English's
exceed. For talking about a value that a temperature doesn't drop below, your best choice is probably 下回る. 上回る is also fine in place of