Term for fonts with fixed versus variable stroke widths
Comparisons between "serif fonts" and "sans-serif fonts" generally compare Times-Roman or Times New Roman against Helvetica or Arial. The presence of serifs on the former and lack of them on the latter, however, is not the most noticeable difference. A more significant difference is the fact that the former fonts are drawn with a mixture of wide and narrow strokes, while the strokes in Arial are of relatively-uniform width. This is most noticeable in letters like "V" which include strokes in both diagonal directions; in Helvetica, the strokes in both directions match, while in Times-Roman they are very different.
Is there a term which would distinguish a font like Belleza:
from Helvetica, or Coustard:
The distinction is perhaps quantitative rather than qualitative, though for fonts where the "V" has diagonal lines with roughly-equal angles, there's probably a pretty clear partition between fonts where the strokes are of equal weight, versus those where they are substantially unbalanced.
As with most typographic terms, there's no single answer to this. So many different terms can go into describing any particular font and the definitions tend to have so much overlap that placing type into defined buckets can be a fruitless undertaking.
Some terms that could apply to what you are looking for.
The term monoline is used for typefaces where every stroke of the letter is the same width. Cursive ball-point-pen typefaces would be a very typical example of this, but any face can be considered monoline if the strokes are all the same width.
This refers to the difference between the thinnest and thickest strokes in your glyphs. A typeface such as Helvetica has very little stroke contrast and would be a 'low contrast' typeface. A blackletter typeface or a Bodoni, would have extreme stroke contrast and be a 'high contrast' typeface.
In regards to your particular examples, note that neither is a true monoline. The difference is going to be with the serifs and the stroke contrast. You could therefore say:
Belleza is a higher contrast humanist sans-serif. Coustard is a lower contrast slab serif.