Before I was chozer b’tshuva, I had some cosmetic surgery in order to remove some excess fat tissue – which caused me embarrassment – from some part of my body. According to the shita that holds that the first born will serve in the Mikdash – would this be considered a blemish and hence disqualify me from the Avodah.
Having the excess fat tissue removed would not render you ‘blemished,’ and please G-d you will be able to serve with your brothers in the rebuilt Mikdash.
Sources: Rambam (Bias Mikdash 6:7) explains that only revealed blemishes are considered a blemish, but somebody with an internal blemish, such as somebody who had his gall bladder removed, is not rendered a ba’al mum. This is all the more true for the removal of excess fat, which does not imply a blemish even in an “internal” sense.