Not crass or vulgar, but has to be done sympathetically.
I've seen a terrible looking, highly spendy bike with non-aero brake routing along the TT, a 1" quill and horizontal dropouts (also steel forks I think?). The last thing it needed was a SRAM Red groupset, but that's what it had (I think...). Mixing vintage frames with Thompson stems and seatposts is always disastrous, fit a Brooks and you've achieved something truly hateful. Quill converters make my heart sink.
On the other hand, someone on the forum (Sidekick?) has a stunning purple Quantum with a Record groupset. Someone on Bikeradar had a really nice lugged steel frame that they'd powdercoated clear and fitted with a DA7800 gruppo, and there's a GT Edge Ti on LFGSS with SRAM and Zipps.
Predictably I think it is harder to get away with the more racey-looking groups - SRAM looks very 'America, f*ck yeah!' and Shimano is hopelessly Battlestar Galactica. But when I had my much-loved Kona Kapu ('98 I think?) I used all sorts of different Campagnolo parts on it - new-shape carbon levers / RD body, new-shape alloy ergos and alloy group, and the previous style ergos... nothing ever looked out of place on that bike.