This is difficult to answer without a picture. Let's see if this helps.
The scores on the parchment are called sirtut and are mandatory on a mezuza (Hilchot Mezuza 1:12). They help to write straight.
R Yerachmiel Askotzky in his book Tefillin and Mezuzos writes (p. 136-137)
If the writing is not in a straight, horizontal line, the parchment is
passul and irreparable according to all poskim. Letters that are written very slightly below the sirtut are acceptable. If there is
writing clearly below the sirtut, some poskim say it is still
kosher as long as the writing is straight. According to other
poskim, writing below the sirtut will make a parchment passul. [...] If the writing is wavy or on an angle, it is passul and irreparable.
In other words as long as letters are within the bounds of - or very slightly below/above - the sirtut then the mezuza is OK. If they are clearly below/above then you get into a machloket.
Net net it is always better to show these questionable parchments to a reputable sofer.