Had a roofer replace some missing ridge tiles on the hip of my roof. He has used mismatching ones. Disregarding aesthetic, is this a problem?
Clay ridge tiles don't really waterproof anything. The mortar joints are common leak points. The tar paper or synthetic membrane underneath does that. The tiles are for protection and appearance.
So, as you suspected, this is just an appearance issue. Tile size aside, I've certainly seen prettier mortar work.
I'd tend to disagree with the other two answers - that appears to be an appallingly bad standard of work. Even a competent DIYer would do a better job than that, let alone someone who claims to be an experienced roofer. I'd be concerned that they're not sufficiently bedded to keep them in place in the long term and that the poor finish will allow water in, which will hasten deterioration by freeze-thaw - particularly where the top tile is raised above the existing. Also, it's not that difficult to find reasonable (not perfect) matches for even very old roof tiles (yours are not ancient - and they're 'modern' concrete, not clay) - again a competent roofer would have little trouble sourcing a much better matching tile. Having said that they'll probably stay up for a few years or more so unless you're planning on staying in the house for a long time you could always just let the next owner worry about it!