GG5, etc.

I don't think naive needs to be explained.

@@ -26 +26,3 @@
-<gloss>naive (i.e. innocent)</gloss>
+<gloss>simpleminded</gloss>
+<gloss>naive</gloss>
+<gloss>unspoiled</gloss>

Shinmeikai says 朴 can also be written 樸, but 樸 and 僕 don't get many counts in the n-grams.

〈純／淳／醇／じゅん／ジュン〉〈朴／樸／僕／ぼく／ボク〉

Google N-gram Corpus Counts
╭─ーーーーー─┬────────┬───────╮
│ 純朴　　　 │ 73,043 │ 98.2% │
│ 淳朴　　　 │    515 │  0.7% │
│ 醇朴　　　 │    121 │  0.2% │
│ じゅん僕　 │     72 │  0.1% │
│ 純樸　　　 │     62 │  0.1% │
│ 淳ぼく　　 │     61 │  0.1% │
│ 淳僕　　　 │     47 │  0.1% │
│ 淳樸　　　 │     41 │  0.1% │
│ 純ぼく　　 │     33 │  0.0% │
│ じゅんぼく │    229 │  0.3% │
│ ジュンボク │    163 │  0.2% │
╰─ーーーーー─┴────────┴───────╯

https://kotobank.jp/jeword/純朴
a naive [an unsophisticated] person


I kind of liked these, I feel like they are sometimes applicable. 
https://eikaiwa.dmm.com/uknow/questions/3054/
Down to earth 
Honest 
Down-to-earth honest personality 
Down-home straightforward personality

"simple-hearted"
https://imperfectatbest.com/tag/happiness/
Let’s see how it can be interpreted. In the first place, we are not simple-hearted anymore. While small children burst into a fit of laughter in the next moment after throwing a tantrum, we, as grownups, like to make things complicated.
一つは、子供のような純朴さを失った。
Is this because of youth or because of devotion to a great cause at the expense of simple, if not naïve, happiness one could enjoy?
若かったからなのか、純朴とは言わないまでも、

homely
BRITISH
(of a place or surroundings) simple but cosy and comfortable, as in one's own home.
"a modern hotel with a homely atmosphere"
NORTH AMERICAN
(of a person) unattractive in appearance.

純朴	73043	96.2%
純朴が	105	0.1%
純朴を	201	0.3%
純朴は	56	0.1%
純朴さが	1269	1.7%
純朴さを	974	1.3%
純朴さは	263	0.3%

純朴さ	6464

I looked for some possibly-more-suitable glosses (and left refs), but I don't think I found anything reliably better.  I mostly just wanted to remove "homely", which for me means "unattractive". 

I don't know what to do if the British and English are in opposition.  Was "homely" from the British English sense perhaps?  There doesn't seem to be anything in this term related to physical appearance.

The "proper" noun is 純朴さ(also listed in sankoku), so removing [n] seems like better advice (even if there are straggling exceptions).

"naive" seems to be perfectly correct, except that there seems to be a strictly positives connotation. If that's the case, we might want to point it out somehow. (I gave "innocent", vs. the colloquial interpretation "stupid" , e.g. "don't be so naive"). 

sankoku doesn't mark it as 文 or anything else, and we have 70k ngrams.  But like a lot of these terms, they aren't necessarily so common.  I came across this funny q&a:
Q: 純朴を使った例文を教えて下さい。 出来るだけ日常で使う自然な表現でお願いします。
https://ja.hinative.com/questions/13916533
A: 日常では使いません。

@@ -24 +23,0 @@
-<pos>&n;</pos>
@@ -27,2 +26 @@
-<gloss>naive</gloss>
-<gloss>homely</gloss>
+<gloss>naive (i.e. innocent)</gloss>

gg5, prog, luminous
純朴	73043	99.1%
淳朴	515	0.7%
醇朴	121	0.2%

@@ -4,3 +3,0 @@
-<k_ele>
-<keb>淳朴</keb>
-</k_ele>
@@ -12,0 +10,4 @@
+<keb>淳朴</keb>
+<ke_inf>&rK;</ke_inf>
+</k_ele>
+<k_ele>
@@ -13,0 +15 @@
+<ke_inf>&rK;</ke_inf>
@@ -23,2 +25 @@
-<gloss>rustic simplicity</gloss>
-<gloss>homeliness</gloss>
+<gloss>simple and honest</gloss>
@@ -27,2 +28 @@
-<gloss>honest</gloss>
-<gloss>simple</gloss>
+<gloss>homely</gloss>