Yes I think we have no proof that Isabel is the daughter of either of the John, Lord Lovel of Titchmarsh.
The Complete Peerage, 2nd ed, Vol 8, pp 217-18 doesn't mention Isabel or Isabella at all. The heir of John Lovel, who died at the Battle of Bannockburn according to the Inquisitions post mortem, was his daughter Joan, aged 2 or 2 and more (the judges not seeming to know about his posthumous son, born in September 1314) - see here and here
It's possible that Isabel is a younger daughter, born in about 1313, but without any mention it's obviously difficult to say for sure.
The ancestry of William Clopton of York County, Virginia has absolutely no sources for the statement, "Isabel, daughter and heir of John Lovell by his wife the daughter of Lord Strange of Blackmere". Even if it was true, then it must be a totally different John Lovell, because we know that she wasn't the heir of either of John Lovel, father and son, we are discussing.
I think we need to find more sources that mention her to know for sure. Even William Calthorp, on Wikitree at least, is without any sources and perhaps that is another avenue to try and find out more information.