is this serious

you may have a faulty fuel sensor or a faulty gauge - l would not say its serious more of a pain really - fill your car up and see what happens otherwise you may run out of diesel somewhere thinking you have a qtr tank left.

Very much as what Sod it said, Take note of the warning light, this did happen to with a van, l took notice of the fuel gauge and ran out of fuel.

Possible a multi relay is faulty.

it sounds like the sender unit may be faulty.
This can be accessed via a large plastic grommet found under the drivers rear seat base.
Once the grommet is removed u will see the top of the sender unit, which is held in by a large plastic nut.

Be careful if u decide to remove the sender unit as the fuel tanks have a habit of distorting when the nut is removed & when u try & put the nut back on it wont screw back on properly.

There have also been issues with the instrument panels on Clio's, but normally when there is a problem with that the trip keeps resetting itself to zero.

No, it is not serious so long as the warning light goes off when there is a lot of fuel in the tank. Most low-fuel lights come on when there is only 2 gallons or so in the tank. In a Clio, that probably is about quarter full. It can also depend if the car is on a slope. l remember in an old Clio l had the fuel guage seemed to be a bit 'approximate' at times, which l was told was due to the odd shape of the tank itself.
If the light is off when there is plenty fuel in it, I'd ignore it.