I assume OP is really looking for a word meaning "stupid", but with positive connotations (along the lines of "cuddly" for "fat", and maybe "homely" for "ugly").
Teachers, for example, may speak of less able students. But that's just because sometimes they can't avoid referring to such pupils (and it would be unprofessional to call them "stupid").
In normal conversational contexts, any reference to someone's lack of intelligence is likely to be seen as impolite and/or hurtful, so it's probably best avoided unless you intend to be disparaging.
OP might feasibly get away with uncomplicated, (or perhaps slow, simple, naive), but generally speaking such terms will be recognised for what they are (euphemisms) - so again, best avoided.
One common approach is to "soften" the label with humour. For example, you can say someone's...
one sandwich short of a picnic (a few clowns short of a circus, etc.)
not playing with a full deck (of cards, similar construction to above)
thick as a brick (...two short planks, ...shit, etc.)
EDIT: Following OP's later comment, it's apparent he wants a friendly/affectionate term by which to address his friend, who's showing a lack of understanding in some particular context. That's not really the same thing as trying to explain to others that your friend is significantly less intelligent than the average.
I suggest suitable phrasing in OP's context would be something slightly "whimsical", such as...
"Oh, you're such a ninny / noodle / nincompoop / etc.! Anyone can see she's madly in love with you!"
Not sure if such a word exists, although it should do. There are several sites which list foreign words which ought to exist in the English language. This one has several suggestions (some words are duplicated). I especially like; toska, torschlusspanik, and tingo.
I was asking my boyfriend, 100% native English speaker who probably reads close to 200 books a year, if he knew of such a word and he suggested schadenfreude, which I said was completely wrong, (in this case) and had a very negative meaning. It's when a person takes delight at somebody's misfortunes or worse, his or her downfall. Which just goes to show being a native speaker and a 100% pure-bred English man is no guarantee that you have a full comprehensive knowledge of the language, either that or he wasn't listening to me.
My first instinct would be to rephrase the sentiment in order to convey that mixed emotion you asked for.
Although happy for her success, [name] couldn't help but feel a
twinge of envy.
Now, why did I change the word jealousy to envy? I don't think a person can be happy and jealous at the same time. Jealousy expresses the negative feeling that one might experience towards an object or person "possessed" by them, i.e. to be jealous of a loved one for example, a girlfriend, a husband, etc. A person who is said to be jealous of his wife is fearful of losing her to a rival; that fear may be either irrational or rational. If you are jealous of your possessions you want to guard them, and make sure no one else can have access to them. Young children are often described as being jealous of their toys.
Envy on the other hand, (for me) is a slightly less negative emotion. I can imagine myself feeling happy for another person's good fortune or success and be unintentionally envious at the same time. I wish I could possess that same object of desire, the same fortune, the same success. I can be envious of a neighbour's new car, a colleague's promotion, or a friend's idyllic family etc.
To lessen or diminish the gravity of that emotion I may add in increasing order of quantity: a bit, a twinge, a pang or a stab.
Best Answer
One phrase to consider might be blissfully ignorant. It doesn't necessarily imply that the person is always "happy", but it describes a person who is content to be unaware of the unpleasant realities of a given situation.
(Edit: I know this is not a single word answer; the original question asked for a word or phrase.)