The one context in which it is appropriate to use the phrase "mostest" is immediately after the words "I really like you the". "Bestest" can be acceptable in this context (and no other). Proofreading concerns are off-matter unless a particular supply of problem in the text is clearly recognized. https://financefeeds.com/bitcoin-holds-above-100k-despite-copyright-market-tensions-btcbull-presale-enters-final-week/