This is an interesting question. My impulse is to say that it is not quite as simple as a gradient between less and more dissonance, for several reasons. First, let's define dissonance as a sonic and syntactic quality that implies tension or potential for harmonic movement. In this sense, the major 7th interval between C and B in the Cmaj7 chord is notably dissonant, because the B "wants to" move upward to C, especially when it is directly juxtaposed with its goal tone in the bass. Now, consider the Cmaj9 chord. On the one hand, we've added an additional non-chord tone to an already-dissonant Cmaj7 harmony. On the other hand, although D is nominally dissonant with respect to the bass tone of C, it is less dissonant than B by virtue of the fact that it is a major second away from C instead of a minor second. Additionally, we've kind of bolstered the standing of B in the harmony because it is now accompanied by a tone a third above in addition to a tone a third below (G), forming a major triad built on G over the C fundamental. I think the fact that the B now participates in a subset of the governing harmony (G major) that is both diatonic and consonant on its own grants it an extra measure of stability not present in the bare Cmaj7 chord. Still, we have to weigh that against a complete accounting of the intervals present in each chord. In a C major chord, we have a minor third, a major third, and a fifth. In a Cmaj7 chord, we have a minor third, two major thirds, two fifths, and a major seventh. In a Cmaj9 chord, we have two minor thirds, two major thirds, three fifths, a minor seventh, and a major seventh. In that sense, we do observe a steady increase in the number of dissonant intervals being sounded at any one time as we continue to stack thirds (none in the C major chord, one major seventh in the Cmaj7 chord, and a minor and major seventh in the Cmaj9 chord). You can easily extend this logic to higher extensions like the 11th (keeping in mind its minor ninth relationship to the third) and 13th.
All that is to say that dissonance, at this point, is complicated. Moreover, it's not just a function of which chord tones are sounding, but also of how you voice the chords in question: a Cmaj11 can sound sweet and ethereal if you voice it (in piano terms) with a C-F-G in the left hand and an E-B-D in the right, but if instead you voice it as C-G-B-D-E-F, it will sound clangorous and off. Note, too, that a Cmaj13 chord doesn't include the 11th, idiomatically speaking, precisely because of the minor ninth between a fundamental chord tone (E) and the extension (F). This is why you usually see Lydian-based voicings on major-ish harmonies when the 11th (or #11th, in this case) is included.
