What is the superlative form of "bonita"?
I know that "-ísimo(a)" is supposed to be added to the end of Spanish adjectives for superlatives, but I'm not sure how bonita is supposed to look...
I would not do superlative form of bonita. I would either say "la mas bonita" or "lindisima"
Normally you would, but not in the case of "bonita", as "bonitísima" sounds cringeworthy. Instead of adding -ísimo/-ísima, just add the word "muy" in front: "muy bonita", or choose another adjective that has a more emphatic meaning than "bonita", like "preciosa" or "hermosa".
Not being a native speaker, 'bonitísima' holds no particular connotation for me. I guess, you'd want to check with someone local to where you were, or local to the audience you were addressing as it may be acceptable in some places.
I live in Colombia and a while back I was invitadísmo to a pizza and glass of wine...