Is an additional heater recommended in the bathroom with underfloor heating?

Yes, Lex is right - using our bathroom on the ground floor as an example:

Heat loss at -12° C [W]: 140
Design temperature [°C]: 24
Heatable area [m²]: 3.7
Theoretically required W/m²: 37.84
Laying distance at 30°/26° [cm]: 5 cm -> results in 13 W/m² (no more possible)
Notes: Heatable area -> 4.7 m² minus shower

One should consider that 30/26 is extremely low for the calculation and -12° only occurs every 10 years. Furthermore, the 13 W assumes that there is a carpet on top and the air cannot circulate freely.
The area simply is not sufficient here - even with a 5 cm laying distance.

You, Yvonne, probably have gas, right?

Yes, we do.

Am I off topic at the moment? Is it about heat pumps? Is there a difference then?

Yes, this thread is about a heat pump - and then you have a very big difference there

Yes, this thread is about a heat pump – and then you have a very big difference there

I only read the headline
Additional heating in the bathroom with underfloor heating
and not every single post again in topics that have been going on for 2 years

in what way is there a difference now?

Insofar as one tries to keep the supply temperatures as low as possible with heat pumps, since every degree more costs about 5% extra. With gas, this is not so dramatic. Therefore, I think one will never have the problem with gas that there is not enough heating surface. Now you have the choice of wall heating, electric auxiliary heating, towel radiator, etc.

... but that doesn't change the fact that gas people do not constantly adjust or change anything about their flow temperature, ok, the flow temperature is basically set higher, who knows....

The towel radiator is basically not intended as an auxiliary heater, no matter what heating system you have
I would rather go for infrared there, it is supposed to emit a nice radiant warmth.