Ideally they should have cut out the rusty area and inserted new metal, then ground the welds flush. This however would cost more.
It looks like they've just plated over the top, leaving some or all of the rusty metal underneath. Whilst this will be strong and good enough for the MOT, if the rust underneath has not been treated completely, it will continue to rust, unseen. More repairs would be needed next year, or the year after.
Whilst this is not cosmetically good, it is adequate. Whether you have a right to complain and ask for a better repair will depend on the terms of the repair. If they offered to "make it ok" for the MOT, that is what they've done. Price will be important. Only you can answer that question, so don't go rushing in complaining.
They should hopefully have given you options for the repair.
1. Adequate for MOT, cheapest price. This is what we see here.
2. Cut out rusty bits and insert repair pieces, grind flush and paint. Cavity wax added after repair. More money and time.
3. Complete new sill. Lots of time and money.
If no options were given, they have done the minimum, probably at the best price. If you want it better, you'll have to visit several potential repairers for quotes.