AFAIK cubase converts mp3's to wav or aiff depending on your project format and then re-encodes it to whatever you set in the export options.

the mp3 format uses fourier methods and comb filters to work out which frequencies are not needed at certain points in the file as they are being masked by other sounds or are too quiet to be heard. Once those sounds are removed they cant be put back. MP3's themselves cannot be editted, hence the conversion.

The big question is which exact method of converting to mp3 did you use before cubase and which method does cubase use. The best mp3 converting is usually acknowledged to be the LAME encoder, but there are probably better other ones, so i suspect the differences between input 128kbs and output 128kbs from cubase is minimal.

you might want to ask on the cubase forums for a more detailled answer.

You could always check by ear as welll, and also 128kbs is shit.