I've done this cab ride multiple times, and $30 is pretty normal from the Downtown Disney area, but Pop Century is further away and the normal route takes you through the high traffic section of Buena Vista in front of Downtown Disney.
Taxifarefinder says $40 with tip from Pop Century to Universal Studios, which feels a bit high, but it might be right.
I'd still take the cab instead of Mears with 4 people. Even if the cab is $50 each way it's so much more convenient that it's worth the extra $10.